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A Boston police officer wheels in injured boy down Boylston Street as medical workers carry an injured runner following an explosion during the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria at the marathon's finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

A Boston police officer wheels in injured boy down Boylston Street as medical workers carry an injured runner following an explosion during the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria at the marathon's finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

AP

A Boston police officer clears Boylston Street following an explosion at the finish line of the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria at the finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

A Boston police officer clears Boylston Street following an explosion at the finish line of the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria at the finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

AP

A flag flies over the finish line as medical workers aid injured people following an explosion at the finish line of the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria at the finish line, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

A flag flies over the finish line as medical workers aid injured people following an explosion at the finish line of the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria at the finish line, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

AP

A photographer captures the moment an explosion goes off close to the finish line of the Boston Marathon

A photographer captures the moment an explosion goes off close to the finish line of the Boston Marathon

Medical workers wheel the injured across the finish line during the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

Medical workers wheel the injured across the finish line during the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

AP

Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. (AP Photo/Charles Krupa)

Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. (AP Photo/Charles Krupa)

AP

Medical workers and authorities work on the scene near the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. (AP Photo/WCVB-TV/ABC) MANDATORY CREDIT

Medical workers and authorities work on the scene near the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. (AP Photo/WCVB-TV/ABC) MANDATORY CREDIT

AP

Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

AP

BOSTON, MA - APRIL 15:  A runner embraces another woman near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: A runner embraces another woman near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  State police guard an area near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: State police guard an area near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  A member of the bomb squad investigates a suspicious item on the road near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: A member of the bomb squad investigates a suspicious item on the road near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Jim Rogash/Getty Images)

BOSTON, MA - APRIL 15: A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  A woman is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Jim Rogash/Getty Images)

BOSTON, MA - APRIL 15: A woman is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Jim Rogash/Getty Images)

BOSTON, MA - APRIL 15: A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Jim Rogash/Getty Images)

BOSTON, MA - APRIL 15: A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Jim Rogash/Getty Images)

BOSTON, MA - APRIL 15: A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Jim Rogash/Getty Images)

BOSTON, MA - APRIL 15: A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  Massachusetts State Police guard an area near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: Massachusetts State Police guard an area near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  Massachusetts State Police guard an area near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: Massachusetts State Police guard an area near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  People gather near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: People gather near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  Runners react near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: Runners react near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  Runners react near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: Runners react near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  Runners react near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: Runners react near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  Boston Police guard an area at the one mile checkpoint near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: Boston Police guard an area at the one mile checkpoint near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  Boston Police guard an area near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: Boston Police guard an area near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  Runner gather near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: Runner gather near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  A runner embraces another woman near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: A runner embraces another woman near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  State police guard an area near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: State police guard an area near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Getty Images

BOSTON, MA - APRIL 15:  A member of the bomb squad investigates a suspicious item on the road near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.  (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: A member of the bomb squad investigates a suspicious item on the road near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Getty Images

A Boston police officer wheels in injured boy down Boylston Street as medical workers carry an injured runner following an explosion during the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria at the marathon's finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/Charles Krupa)

A further two bombs have been found close to the scene of two blasts in Boston.

A senior US intelligence official said two other explosive devices were found nearby the scene of initial explosions close to the finish line at the Boston Marathon.

A third explosion was heard about an hour after the first two after authorities warned spectators to expect a loud noise from a water cannon that police apparently were using to destroy one of the devices.

Shortly after the explosions, Secret Service shut down Pennsylvania Avenue outside the White House, cordoning off the area with yellow police tape. Several Secret Service patrol cars also blocked off the entry points to the road.

The White House was not on lockdown and tourists and other onlookers were still able to be in the park across the street from the executive mansion.

The Federal Aviation Administration warned pilots that it had created a no-fly zone over the site of the explosions in Boston.

The agency said in a notice issued Monday about an hour after the explosions that a no-fly zone with a 3.5-mile (5.6-km) radius has been created over 811 Boylston Street. The zone is limited to flights under 3,000 feet (914 metres), which is lower than most airliners would fly except when taking off or landing.

The notice said the no-fly zone is effective immediately and will remain in effect until further notice. Pilots planning flights were urged to call their local flight service station.

Homeland Security Secretary Janet Napolitano directed her agency to provide "whatever assistance" necessary.

Belfast Telegraph