“Clearly they were equipped and they could have done another attack,” Chief Burguan said. “We intercepted them before that happened, obviously.”

With investigators seeking a motive and collecting evidence, law enforcement officials raided the townhouse apartment of Syed Raheel Farook, the dead suspect’s brother, according to neighbors. The F.B.I. also combed the background of the suspects.

The F.B.I. uncovered evidence that Mr. Farook communicated with extremists, domestically and abroad, a few years ago, but not recently, according to congressional officials who were briefed on the investigation.

They said he had contacts with five people whom the F.B.I. had investigated for possible terrorist activities — including one associated with the Shabab, the Islamist militant movement in Somalia, and another associated with the Nusra Front, the wing of Al Qaeda in Syria. In all five cases, the investigations were closed and no charges were filed.