UPDATE | Police looking for possible 3rd suspect in Calif. mass shooting

SAN BERNARDINO, Calif. (AP) — As many as three gunmen believed to be wearing military-style gear opened fire today at a Southern California social services center "as if they were on a mission," killing at least 14 people and seriously wounding more than a dozen others, authorities said.

Hours later, police hunting for the attackers riddled a black SUV with gunfire several miles away, and one person lay motionless in the street — dead or dying — with a gun nearby. Officers appeared to remove a second person from the vehicle.

San Bernardino police spokeswoman Sgt. Vicki Cervantes said authorities had not immediately confirmed whether those in the SUV were involved in the morning carnage. As darkness fell, law officers swarmed a neighborhood about a mile from the SUV, apparently in a hunt for a possible third gunman.

It was the nation's deadliest mass shooting since the Newtown, Conn., attack in December 2012 that left 26 children and adults dead.

Police shed no light on a motive for today's massacre, which came just five days after a gunman opened fire at Planned Parenthood in Colorado, killing three.

In what authorities described as a carefully planned assault, the gunmen invaded the Inland Regional Center, which serves people with developmental disabilities, and began shooting around 11 a.m. They opened fire in a conference area that the San Bernardino County Department of Public Health had rented out for a banquet, said Marybeth Feild, president and CEO of the center.

"They came prepared to do what they did, as if they were on a mission," San Bernardino Police Chief Jarrod Burguan said, noting the attackers carried long guns — which can mean rifles or shotguns.

FBI agents and other law enforcement authorities converged on the center and searched room to room for the attackers, but they had apparently escaped.