Mr Obama told the Spanish-language television network Univision that he does not think it is fair to say law enforcement “dropped the ball”.
He 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.”.The President added that it was a challenge when would-be attackers were “self-radicalising” and not part of a broad conspiracy.
Mr Obama said the FBI “can’t arrest somebody just based on a rumour”.
The first of a series of House hearings is set for Thursday before the GOP-led House Homeland Security Committee.
Legal experts says authorities are placing intense pressure on bomb suspect Tamerlan Tsarnaev’s widow Katherine Russell and three of his brother’s detained friends to co-operate in the investigation.
Federal agents follow Russell 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 remanded on charges related to covering up his activities.
The experts say investigators are trying not just to determine whether 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.
Security was heightened for yesterday’s Pittsburgh Marathon, which featured about three dozen runners who were unable to finish last month’s Boston Marathon owing to the bombings.