Wednesday's mass shooting at a Southern California centre for the developmentally disabled came on the heels of other United States (US) massacres but differed from most in key ways, including the involvement of multiple people, including a woman, and an apparently well-planned escape route.

As night fell in the San Bernardino area near the site of the attack that killed 14, police said a man and a woman dressed in tactical gear died in a gunfight with police and a third was in custody, as the Federal Bureau of Investigation said the agency was considering the possibility that the shooting could be a terrorist attack.

"These are people that came prepared," San Bernardino Police Chief Jarrod Burguan said in a news conference. "They were dressed and equipped in a way that indicates they were prepared. They were armed with long guns, not handguns."

The attack in San Bernardino, a city of about 200,000 about 100 km east of Los Angeles, was perplexing, as police struggled to determine a motive and to find the shooters, who had fled in a black SUV.

It did not bear the hallmarks of a lone-wolf killing, yet differed in key ways from attacks like those perpetrated by known militant groups, US officials said.

For example, these officials said, the terror attacks in Paris last month deliberately focused on public places where carnage would have the biggest shock value and frighten residents the most.

By contrast, Wednesday's attack took place in a little-known location and was more likely to affect families of the disabled and county employees.

The San Bernardino shooting came just days after a shooter opened fire in a Planned Parenthood clinic in Colorado, killing three people and wounding nine others before surrendering to police after a bloody siege lasting several hours inside the facility.

In October, a gunman killed his English professor and eight others at a community college in Oregon, and in August, a television reporter and cameraman were fatally ambushed by a former employee of their Roanoke, Virginia, television station while they were interviewing a woman on live TV.

All told, there have been about 350 mass shootings in the US so far in 2015, defined as incidents in which four or more people were shot, according to the crowd-sourced website shootingtracker.com, which keeps a running tally of mass shootings.

But whereas in most shooting incidents involved just one shooter ─ a man who frequently killed himself or was killed by police at the scene ─ Wednesday's massacre involved at minimum a man and a woman, and police said that a third suspect was also in custody.

Authorities were also not ruling out the involvement of additional people in planning the event and the shooters' escape, police said.

A study of 160 active shooter situations between 2000 and 2013 by the Federal Bureau of Investigation showed that all but two involved a single shooter. Just six involved women.

By evening, authorities had not named any of the suspects, despite reports by some news agencies, unconfirmed by Reuters, that one of them had a Middle Eastern-sounding name.

David Bowdich, assistant director of the FBI's Los Angeles office, said that while investigators had initially ruled out terrorism as a motivation for the attack, they were now considering it.

"It is a possibility but we don't know yet," he said at a news conference. "It's incredibly fluid."