Police officers conduct a manhunt after a shooting rampage in San Bernardino, California December 2, 2015. [Photo/Agencies]

SAN BERNARDINO, Calif. - A man and woman armed with assault-style rifles opened fire on the holiday party of his co-workers in Southern California, killing 14 people and wounding 17 others, and then were slain hours later in a shootout with police, authorities said.

The two suspects were identified as Syed Rizwan Farook, 28, and Tashfeen Malik, 27, who San Bernardino Police Chief Jarrod Burguan said were in a relationship, possibly married or engaged. He said they were believed to be the only shooters involved in an attack that required some degree of planning.

While the motive remained unclear, Burguan said, "We have not ruled out terrorism." Farook was US born, while Malik's nationality was still undetermined.

The shooting rampage marked the deadliest US 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.

Wednesday's carnage amplified concerns about gun violence and security in the wake of a deadly rampage on a Planned Parenthood clinic in Colorado Springs last week and the attacks in Paris three weeks ago by Islamic State militants that killed 130 people.

So far in 2015, there have been more than 350 shootings in which four or more people were wounded or killed, according to the crowd-sourced website shootingtracker.com, which keeps a running tally of US gun violence.