President Barack Obama has defended the FBI's handling of the Boston Marathon bombing investigation as Republicans prepared for oversight hearings on the attack.
The president said: "There are going to be times where individuals decide they want to cause harm to people for crazy reasons, for no good reason, for ideological reasons".
Authorities are placing intense pressure on the widow of one Boston Marathon bomber and three detained friends of the other to co-operate in the ongoing investigation, according to legal experts.
Federal agents follow the widow of Tamerlan Tsarnaev in unmarked vehicles when she leaves her parents' Rhode Island home, it has been claimed.
College friends of surviving suspect Dzokhar Tsarnaev face more than five years in prison after being jailed on charges related to covering up his activities.
The experts say investigators are trying not just to determine whether widow Katherine Russell and the friends are culpable, but also to force them to share all they know about the brothers and possibly a wider plot.
Russell's lawyers said she is speaking with FBI agents. The students' attorneys say their clients did not know about the plot.