A man and woman armed with assault-style rifles opened fire on a 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."

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Farook was U.S. born, while Malik's nationality was still undetermined. They were parents of a six-month-old baby girl.

Farook's family and co-workers struggled to make sense of the shooting, the deadliest in the United States in three years. His brother-in-law went before television cameras and said he had "absolutely no idea" why Farook would stage a massacre.

According to the police chief, Farook was a county public health employee who attended the party, held in a conference building on the campus of the Inland Regional Center - a social services agency – and at some point stormed out. He returned with Malik to open fire on the celebration. The couple were dressed in assault-style clothing and also placed several bombs at the scene, which police detonated.

The shooting rampage 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.

So far in 2015, the United States has seen 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 U.S. gun violence.

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The attack in San Bernardino, a largely working-class city 60 miles (100 km) east of Los Angeles, appeared to differ from other recent U.S. killing sprees in several ways, including the involvement of two people rather than a lone perpetrator.

A third person seen fleeing from the area of the shootout with the suspects was detained, but Burguan said he was not sure if that person was involved in the attack.

David Bowdich, an assistant regional FBI director, said earlier in the day that authorities had not yet determined whether the shooting was an act of terrorism.

"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."