Both U.S. officials are close to the ongoing investigation but spoke on condition of anonymity because they were not authorized to discuss it publicly.
It was not immediately clear what the detonation device was.
A criminal complaint outlining federal charges against the surviving bombing suspect, Dzhokhar Tsarnaev (joh-KHAHR' tsahr-NEYE'-ehv), described him as holding a cellphone in his hand minutes before the first explosion.
Cellphones have been used to trigger bombings in war zones.
Copyright 2013 The Associated Press.