The man and woman, Syed Rizwan Farook, 28, and Tashfeen Malik, 27, were killed in an exchange of fire with police.
Farook had been a San Bernardino public health employee for five years, police chief Jarrod Burguan said.
The attack took place at an event on Wednesday for Farook’s colleagues. None of the victims has been identified.
No motive has been established, but the FBI is considering both workplace violence and terrorism, according to David Bowdich from the bureau’s Los Angeles office.
The Inland Regional Center, where the shooting took place, provides services for people with developmental disabilities, but the shooting appeared to be unrelated to its clients.
The incident marks the deadliest mass shooting in the US since 26 people were killed at a school in Newtown, Connecticut in 2012.
Mr Burguan, who said another 17 people were wounded, said police were “reasonably confident” that there were just two shooters.
A third person seen running from the scene was detained, but police said it was unclear whether that person was involved.
Both suspects were killed in their vehicle after a shoot-out with police involving 20 officers. A tip-off had earlier led police to a house in nearby Redlands.
The dead man and woman were believed to be a couple, the police chief said. They were armed with assault rifles and semi-automatic handguns and wore military-style clothing.
The authorities said all the weapons had been purchased legally but did not explain how they got into the hands of the perpetrators.
One police officer was injured, but his injuries are not life threatening.
Three possible explosive devices were found at the Inland Regional Center, where the shooting took place.
Mr Burguan said that Farook had left the event – which was possibly a holiday party for employees or a meeting – “under some circumstances that were described as angry,” and returned with Malik.
San Bernardino is a city of about 200,000 people, 60 miles (100 km) east of Los Angeles.
At around 11:00 local time (19:00 GMT) on Wednesday, the shooting began at the social services centre. Hundreds of people were evacuated as police officers swept through the massive complex.
Others locked themselves in rooms waiting for police to arrive.
“People shot. In the office waiting for cops. Pray for us. I am locked in an office,” Terry Petit’s daughter said in a text message to him from inside the centre.
The attack took place in a conference area where the San Bernardino County Department of Public Health was holding a social event, said Maybeth Field, the centre’s president.
Farook, who was born in the US, worked for the county as an environmental health specialist, police said. They had no information on whether he had a criminal record, nor further details on Malik.
Colleagues told the Los Angeles Times that Farook had travelled to Saudi Arabia and returned with a new wife. The couple had a young baby, they said.
The pair had dropped off their infant with relatives early on Wednesday, Hussam Ayloush of the Council on American-Islamic Relations said after talking with family.
Farook’s brother-in-law Farhan Khan told a press conference he had “absolutely no idea” what could have prompted the shooting.
The mass shooting comes just days after three people were killed at a Colorado Planned Parenthood clinic.
US President Barack Obama said in response to the shooting: “One thing we do know is that we have a pattern of mass shootings in this country that has no parallel anywhere in the world.
“There are some steps that we could take, not to eliminate all incidents, but to make sure they happen less frequently.”
Source: BBC
]]>