San Bernardino attackers left remote-controlled bomb at party

Authorities said there were three pipe bombs tied together in a bag at the scene, and the design indicated that suspects Syed Farook and Tashfeen Malik appeared to believe their initial attack would draw a heavy law enforcement and emergency personnel response and left behind a device that would cause still greater casualties.

The device did not explode, and investigators do not know if the controller was too far away to trigger the bomb. Forensic investigation of the device has not been completed.

The attackers had also prepared metal pipe bombs of the type used by the Tsarnaev brothers after the Boston Marathon bombing. San Bernardino Police Department Chief Jarrod Burguan said Thursday at a press conference that 12 pipe bombs were found at a residence in nearby Redlands. Though there is no known link between the Boston and San Bernardino incidents, the California attackers and the Tsarnaev brothers seemed to be using designs found in multiple issues of Inspire magazine, the on-line publication of al Qaeda in the Arabian Peninsula.

Authorities are investigating how the San Bernardino attackers learned to construct their devices, and are looking for any possible accomplices who might have helped them obtain the large supply of ammunition, bombmaking materials and any weapons that may not have been directly purchased by the suspects.

More forensic evidence related to bomb-making was found inside the Redlands residence. Authorities told NBC News that investigators found traces of explosive residue indicating that the bombs may have been built there.