Mr. Farook went to the holiday luncheon for employees of the county health department, where he worked as a health inspector, left early — agitated, according to some witnesses — and returned a short time later with his wife, both in tactical gear and with an arsenal to mow down his co-workers. But Chief Jarrod Burguan of the San Bernardino police said there was more at work than a spontaneous workplace dispute.

“There appears to be a degree of planning that goes into this,” the chief said at a news conference. “Nobody goes to a party and puts together such an elaborate scheme.”

Both at the center, and hours later on a quiet residential street where Mr. Farook and Ms. Malik died in a shootout with the police, this city was rocked by a barrage of gunfire more typical of a war zone.

The suspects fired 65 to 75 rifle rounds inside the center, leaving behind four spent high-capacity magazines. Hours later, cornered after a chase, “the suspects are believed to have fired 76 rifle rounds at the officers,” Chief Burguan said.