Law enforcement officials said the F.B.I. had uncovered evidence that Mr. Farook was in contact over several years with extremists domestically and abroad, including at least one person in the United States who was investigated for suspected terrorism by federal authorities in recent years. The officials spoke on the condition of anonymity because they were not authorized to discuss the matter publicly.
The officials called the case perplexing, saying that no clear evidence of terrorism had emerged, though the attack was clearly premeditated. The victims were Mr. Farook’s co-workers at the county health department, and the shooting may have involved grievances against them, but it did not fit the mold for workplace violence, either.
The idea that this was a workplace argument that spiraled out of control seems far-fetched, the officials said, given the explosives and the preparation. “You don’t take your wife to a workplace shooting, and especially not as prepared as they were,” said a senior law enforcement official briefed on the investigation. “He could have been radicalized, ready to go with some type of attack, and then had a dispute at work and decided to do something.”