The capture of a second suspect in the Boston Marathon bombings "closed an important chapter in this tragedy," US President Barack Obama has said.
But he acknowledged that many unanswered questions remain about the motivations of the two men accused of perpetrating the attacks that unnerved the nation.
"The families of those killed so senselessly deserve answers," said Mr Obama, who branded the suspects "terrorists".
The president spoke from the White House briefing room just over an hour after law enforcement officials apprehended 19-year-old Dzhokhar Tsarnaev.
His capture capped a frenzied week in Boston, Washington and elsewhere around the country.
Letters addressed to Mr Obama and Republican Senator Roger Wicker of Mississippi were found to contain traces of poisonous ricin in tests. That evoked eerie parallels to the anthrax attacks that followed the terror attacks of September 11, 2001. And a massive explosion levelled a Texas fertiliser plant, leaving at least 14 people dead and more than 200 injured.
In his remarks, Mr Obama told the people of West, Texas, that "they are not forgotten". But he focused the bulk of his comments on the day's events in Massachusetts, saying the nation owed a debt of gratitude to law enforcement officials and the city of Boston.
Mr Obama urged the public against rushing to judgment as officials seek answers to the many questions that remain.
"When a tragedy like this happens, with public safety at risk and the stakes so high, it's important that we do this right," he said. "That's why we take care not to rush to judgment - not about the motivations of these individuals, certainly not about entire groups of people."
The president's praise for law enforcement was echoed by Republican leaders in Congress. House of Representatives Speaker John Boehner of Ohio called their actions "a job well done under trying circumstances". And Senate Minority Leader Mitch McConnell of Kentucky said the US "marvelled at the co-ordination, skill, and bravery of military, intelligence, and law enforcement officials at every level".