Reuters/San Bernardino

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."

Farook was US 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.

Wednesday's carnage amplified concerns about gun violence and security after deadly assaults at 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, 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.

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."

'WHY WOULD HE DO THAT?'

At a news conference called by the Los Angeles area chapter of the Muslim advocacy group Council on American-Islamic Relations (CAIR), the brother-in-law of Farook, Farhan Khan, said he was bewildered by the news.

"Why would he do that? Why would he do something like this? I have absolutely no idea. I am in shock myself," Khan said at the news conference in Anaheim, California, south of Los Angeles.

Co-workers told the Los Angeles Times they were surprised to hear Farook's name linked to the shootings since he was quiet and polite and did not bear any obvious grudges. They also said he had traveled to Saudi Arabia and returned with his new wife whom he had met online.

Hussam Ayloush, executive director of CAIR in the Los Angeles area, appealed to the public not to jump to conclusions about the suspects' motives.

"Is it work? Is it rage-related? Is it mental illness? Is it extreme ideology?" he said. "We just don't know."

The manhunt initially led police to a home in the neighboring town of Redlands. Police then pursued a suspected getaway vehicle that was seen leaving that address back to San Bernardino, where the shootout occurred.

Ayloush told Reuters the couple left their six-month-old-baby with Farook's mother at that Redlands home early on Wednesday morning and told her they were going to attend a doctor's appointment for Malik, whom he called Farook's wife of two years.

Twelve hours after the attack, the identities of the dead had still not been made public as investigators delayed examining the crime scene because of the explosives left there, Burguan said.

Two adult victims of the shooting were in critical but stable condition at Loma Linda University Medical Center, which said in a statement that it had received five patients hurt in the attack. The conditions of patients taken to other medical facilities was not immediately known.

The shooting and ensuing manhunt practically paralyzed San Bernardino, a city of 220,000 that was hard hit by the housing foreclosure crisis and later declared bankruptcy. Dozens of schools, public offices and hospitals were placed on security alert for hours.

President Barack Obama lamented an epidemic of gun violence that he said "has no parallel anywhere else in the world."

And he repeated his call for Congress to pass "common sense gun safety laws," including tougher background checks for firearm sales.