SAN BERNARDINO, California (REUTERS) - Authorities on Thursday (Dec 3) were working to determine why a man and a woman opened fire at a holiday party of his co-workers in Southern California, killing 14 people and wounding 17 in an attack that appeared to have been planned.

Syed Rizwan Farook, 28, and Tashfeen Malik, 27, who had a 6-month-old daughter together, were killed in a shootout with police after Wednesday's (Dec 2) bloodshed at the Inland Regional Center in the city of San Bernardino, a social services agency where Farook worked as an inspector.

San Bernardino Police Chief Jarrod Burguan said the motive remained unclear in the most recent US mass shooting. He and David Bowdich, an assistant regional FBI director, both said it had not been determined if terrorism was a motive.

The guns used in the attack were legally purchased in the United States, the Associated Press reported, citing a spokeswoman for the US Bureau of Alcohol, Tobacco, Firearms and Explosives.

Farook was US born, while Malik's nationality was yet to be determined.

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.

Burguan said Farook was a county public health employee who attended the party, held in a conference building on the campus of the Inland Regional Center 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.

'DEGREE OF PLANNING'

Burguan said the manner in which the couple was equipped indicated there was "some degree of planning" behind the attack.

Meredith Davis, a spokeswoman for the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), said the two assault-style rifles and two handguns recovered at the scene of the shootout were bought legally in the United States.

She said two of the guns were purchased by someone"associated with this investigation," while the buyer of the other two was not linked to the investigation.

The long guns were .223-calibre and their ammunition can go through protective vests and walls, Davis said.

There have been more than 350 shootings this year, in which four or more people were wounded or killed in the United States, according to the crowd-sourced website shootingtracker.com, which keeps a running tally of US gun violence.

The San Bernardino rampage was the deadliest US gun incident since the massacre at Sandy Hook Elementary School in Newtown, Connecticut, in Dec 2012, in which 27 people, including the gunman, were killed.

"I don't think any community is immune," San Bernardino Mayor Carey Davis told CBS. "Certainly, we don't anticipate that kind of thing happening here. It was a shock."

Wednesday's (Dec 2) 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 in Iraq and Syria (ISIS) militants that killed 130 people.

President Barack Obama called for gun law reform to reduce the likelihood of mass shootings. "We have a no-fly list where people can't get on planes but those same people who we don't allow to fly could go into a store right now in the United States and buy a firearm and there's nothing that we can do to stop them," he said in an interview with CBS News on Thursday (Dec 3).

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

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

'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 told the newspaper he had travelled 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 vehicle that was seen leaving that address back to San Bernardino, where the shootout occurred.

Ayloush told Reuters the couple left their baby with Farook's mother at that Redlands home early Wednesday (Dec 2) and told her they were going to a doctor's appointment for Malik, whom he described as Farook's wife of two years.

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