The centre provides social services for people with developmental disabilities. However, the conference room had been hired by the outside group.
The shooters - wearing black, with their faces obscured by ski masks - left just as soon as they had arrived; fleeing in a dark-coloured SUV as authorities were notified.
One of the suspects was identified as Syed Farook, 28, an American citizen.
His family told media last night they were shocked of his involvement in the attack.
Relative Farhan Khan said: "I have absolutely no idea why he would do this. I am shocked myself."
A second suspect was Tashfeen Malik, 27, believed to be Farook's wife.
The couple had a child together, born this year.
Both died less than five hours after the shooting during a shoot-out with police, in which an officer was also shot. He is expected to survive.
A third person was taken into custody, but was not a shooter.
The motive for the massacre remained unclear last night, but a policeman told the Los Angeles Times it was understood one of the shooters had had a row with a co-worker.
A co-worker of Farook's, Patrick Baccari, confirmed Farook had been at the party earlier in the day and had been sitting at the same table as him.
Farook suddenly disappeared - leaving his coat on his chair. Mr Baccari said the reserved Farook showed no signs of unusual behaviour, although he grew out his beard several months ago.
Mr Baccari had stepped into the bathroom when the shooting started and suffered minor wounds from shrapnel slicing through the wall.
Survivor stories paint a horrifying picture of what happened.
Denise Peraza, 27, said the doors opened and two people in black and wearing face masks entered with "big ol' guns" and started shooting.
"Everyone dropped to the floor. The guys opened fired for 30 seconds, randomly, then paused to reload and began firing again."
As she cowered under a desk, she was shot in the lower back.
President targets gun laws
United States President Barack Obama responded to the San Bernardino massacre with a repeated call for tighter gun-control laws.
"The one thing we do know is that we have a pattern now of mass shootings in this country that has no parallel anywhere else in the world," Obama told CBS.
He said there were "steps we can take, not to eliminate every one of these mass shootings, but to improve the odds that they don't happen as frequently".
These could include common-sense gun safety laws and stronger background checks, he said.
"We should come together in a bipartisan basis at every level of Government to make these rare as opposed to normal," Obama said.
"We should never think this is something that just happens in the ordinary course of events because it doesn't happen with the same frequency in other countries."
He has delivered remarks 15 times after mass shootings in the US, the most recent one after the shooting at Umpqua Community College in Oregon on October 1.
Mass shootings have become so pervasive that Hillary Clinton was actually talking about guns in Orlando minutes before yesterday's tragedy unfolded. "It is time for us to say we are going to have comprehensive background checks, we are gonna close the gun show loopholes," Clinton said.
But most Republicans say there's no proof that more gun laws would do any good, and some even want more firearms.
"In most every case that we've had in this country, two things have been common denominators. One, a mentally unstable person. Two, a gun-free zone," said Republican presidential hopeful Mike Huckabee.
