President Barack Obama vowed to find out what turned two young US residents accused of the Boston bombings to violence, after the second suspect was captured alive, ending a week of horror.
Obama said in a late-night on-camera statement that the attacks had failed because Americans refused to be terrorized and heaped praise on police and law enforcement services while remembering the dead and injured.
"Tonight, there are still many unanswered questions. Among them, why did young men who grew up and studied here as part of our communities and our country resort to such violence?" Obama asked.
"How did they plan and carry out these attacks? And did they receive any help? The families of those killed so senselessly deserve answers."
The president came to the podium in the White House press briefing room shortly after law enforcement services captured suspect Dzhokhar Tsarnaev, 19, alive in the Watertown suburb of Boston.
Meanwhile, Russia and the United States yesterday agreed to step up cooperation in their fight against terror in the wake of news that two ethnic Chechens were suspected of organising the deadly Boston Marathon bombings.
The Kremlin said Russian leader Vladimir Putin called US President Barack Obama to once again express his condolences and discuss ways the two sides can work more closely on security in the runup to the 2014 Winter Olympic Games in Sochi.