Almost a million residents in Boston were warned to stay indoors as thousands of armed police combed the city in a massive hunt for marathon bombing suspect Dzhokhar Tsarnaev. The ethnic Chechen's elder brother, Tamerlan, was killed in a shootout with police earlier after a night of violence in which the pair killed a university police officer and hurled explosives at police in a car chase and gun battle.
Boston is in lockdown with public transport shut and transport to and from the area is virtually shut down, with the exception of aircraft. Police SWAT teams, sharpshooters and FBI agents with armoured vehicles surrounded various buildings as police helicopters buzzed overhead.
The brothers from Dagestan, which neighbours Chechnya in southern Russia, have lived near Boston and had been in the US for about a decade, their uncle Ruslan Tsarni, of Montgomery Village, Maryland, said. He urged Dzhokhar to give himself up and described the pair as barbarians but their father Anzor Tsarnaev said Dzhokhar was a "true angel" and claimed they had been "set up".
Tamerlan Tsarnaev, 26, known to the FBI as Suspect No 1 was seen in surveillance footage in a black baseball hat. His 19-year-old brother - dubbed Suspect No. 2 was seen wearing a white, backward baseball cap in the images from Monday's deadly bombing at the marathon finish line - escaped.
Authorities gave no details on how Dzhokhar Tsarnaev escaped, but said he may have been in a Honda CRV that was found later in the morning in Boston. "We believe this man to be a terrorist," said Boston Police Commissioner Ed Davis. "We believe this to be a man who's come here to kill people."
The bombings on Monday killed three people and wounded more than 180 others, instantly raising the spectre of another terrorist attack on US soil.
The endgame - at least for Suspect No. 1 - came just hours after the FBI released photos and video of the two young men at the finish line and appealed to the public for help in identifying and capturing them. Tips came pouring in to the FBI immediately.
The city of Cambridge announced two years ago that it had awarded a 2,500 (£1,600) scholarship to Dzhokar Tsarnaev, who was listed as a senior at Cambridge Rindge & Latin School, a highly regarded public school whose alumni include Matt Damon, Ben Affleck and NBA star Patrick Ewing.
The images released by the FBI depict the two young men walking one behind the other near the finish line. Richard DesLauriers, FBI agent in charge in Boston, said Suspect No. 2 in the white hat was seen setting down a bag at the site of the second of two deadly explosions.
Authorities said surveillance tape recorded late at night showed Suspect No. 2 during a robbery of a convenience store in Cambridge, near the campus of the Massachusetts Institute of Technology, where university police officer Sean Collier, 26, was shot dead while responding to a report of a disturbance. From there a deadly car chase ended in Watertown where witnesses reported hearing multiple gunshots and explosions.