Authorities in the US are struggling to work out why a man and woman reportedly opened fire after a dispute at his employer’s Christmas party.
Syed Rizwan Farook and Tashfeen Malik were killed in a shootout with police after Wednesday’s massacre in which 14 people were killed.
US President Barack Obama ordered flags flown at half-mast in memory of the victims.
“It is possible that this was terrorist-related. But we don’t know. It is also possible that this was workplace-related,” said the president.
“We going to have to search ourselves as a society to make sure we can take basic steps that will make it harder not impossible but harder for individuals to get access to weapons,” added Obama.
The mass shooting took place in the city of San Bernardino at a social services agency where Farook worked as an inspector.
Local media reported co-workers describing him as quiet and polite.
The US House Democratic Leader Nancy Pelosi has backed congressional efforts to address mental illness, saying mass shootings this week highlight the need for additional action.