The shooting rampage at a holiday party on the campus of a social services agency marked the deadliest U.S. gun violence since the massacre at Sandy Hook Elementary School in Newtown, Connecticut, in December 2012, in which 27 people, including the gunman, were killed.

Authorities also detained an individual seen running away from the vehicle, but investigators were not immediately sure that person was involved in the case, Police Chief Jarrod Burguan said at a news conference.

The chief said it was possible that a third shooter remained at large and that there were other people "involved in the planning" of the crime.

Burguan said the two suspects who were killed were armed with assault rifles and handguns and were dressed in "assault-style" clothing.

The police chief said he knew of no possible motive for the attack.

"It is a possibility, but we don't know that," he told reporters. "It's possible it goes down that road. It's possible it does not."

Burguan said earlier, "Obviously, at a minimum, we have a domestic-type, terrorist-type situation that occurred here."

NBC News reported one of the three suspects was identified by multiple sources as Syed Farook, but Reuters was not able to confirm that.

The attack took place on the campus of the Inland Regional Center, an agency that serves the developmentally disabled, in a building housing a conference center that was being used for the holiday celebration, authorities said.

The Los Angeles Times, citing information from a senior federal official who was monitoring the case, reported that investigators believe one of the shooters left the party after getting into an argument and returned with one or two armed companions.

Burguan said he was aware someone left the party following a dispute but did not know whether that individual returned. The chief said he knew of no possible motive for the shooting spree.

Burguan said the manhunt initially led police to a home in the neighboring town of Redlands, and that police pursued a suspected getaway vehicle that was seen leaving that address back to San Bernardino, where the shootout ensued.

Bomb disposal technicians were examining a number of suspicious items left by the armed assailants at the Inland Regional Center, including one that was "believed to be a potentially explosive device."

The FBI's Bowdich said federal agents and local law enforcement were being cautious about entering the house in Redlands because of concerns about explosives that might have been left there.