The suspected killers died hours later in a shoot-out with police. They were named as Syed Rizwan Farook, 28, and his wife or fiancee Tashfeen Malik, 27.
Farook was born in the United States.
Wednesday’s shooting happened at a social services centre for the disabled which had been rented by Farook’s colleagues in the San Bernardino County Department of Public Health for a celebration.
It was America’s deadliest mass shooting since the attack at a school in Newtown, Connecticut, three years ago that left 26 children and adults dead.
The attackers burst into the Inland Regional Centre about 60 miles east of Los Angeles, and opened fire.
“They came prepared to do what they did, as if they were on a mission,” police chief Jarrod Burguan said.
Farook attended the event before leaving. Colleague Patrick Baccari said he was sitting at the same table as Farook, who suddenly disappeared, leaving his coat on his chair. Mr Baccari said when the shooting started, he hid in a bathroom and suffered minor wounds from shrapnel. Mr Baccari described Farook as reserved and said he showed no signs of unusual behaviour. Earlier this year he travelled to Saudi Arabia and returned with a wife, Mr Baccari said.
The FBI is investigating several possible motives.
Farook was a restaurant inspector for the health department and had been a county employee for five years.
The couple dropped off their six-month-old daughter with relatives on Wednesday morning, saying they had a doctor’s appointment, Hussam Ayloush, executive director of the Council on American-Islamic Relations, said after talking with family.
Farhan Khan, who is married to Farook’s sister, said he last spoke to him about a week ago. He said he was in shock, condemned the violence, and had “absolutely no idea why he would do this.”