The Boston Marathon explosions which killed three people and injured more than 260 were triggered by a remote-controlled detonator, US officials have said.
They said the bombs were not very sophisticated. One described the detonator as "close-controlled" - meaning it had to be triggered within several blocks of the bombs.
It was not immediately clear what the detonation device was.
A criminal complaint outlining federal charges against the surviving bombing suspect, Dzhokhar Tsarnaev, described him as holding a mobile phone in his hand minutes before the first explosion.
Mobile phones have been used to trigger bombings in war zones.