Authorities believe it was intended to be set off by remote control by a cell phone found inside. The cell phone had received at least two calls, which should have detonated several gallons of gasoline, but when the calls came in, the bomb failed to go off, the official said.

Had it done so, that blast then would have ignited six to eight tanks of propane in a mist to make a fuel-air explosion, creating a fireball the size of a small house and propelling 18 to 20 boxes of roofing nails around a large area at bullet speed, counterterrorism officials said.