US officials say the Boston Marathon explosions that killed three people and wounded more than 260 were triggered by remote-control.
Two officials said the bombs were not very sophisticated.
One of the officials described the detonator as “close-controlled” — meaning it had to be triggered within several blocks of the bombs.
Both US officials are close to the probe, but spoke on condition of anonymity. It was not immediately clear what the detonation device was.
Meanwhile hundreds of police attended a memorial service at the Massachusetts Institute of Technology for officer Sean Collier, allegedly shot by the bombing suspects.