US police are trying to establish why a man and woman opened fire at a social services centre in California, killing 14 people and wounding 17 more.
The pair, named as Syed Rizwan Farook, 28, and Tashfeen Malik, 27, were killed in an exchange of fire with police.
President Obama said it was "just too easy" for individuals to carry out such mass shootings in the US.
He said it was possible the latest incident was terror-related but the authorities do not know at this stage.
The FBI is investigating terrorism and a workplace grudge as possible motives, the bureau's Los Angeles office said earlier.
The incident took place at the Inland Regional Center in San Bernardino, where Farook worked.
It marks the deadliest mass shooting in the US since 26 people were killed at a school in Newtown, Connecticut in 2012.
Farook and Malik are thought to be a couple with a six-month-old baby. Police said the attack indicated there had been "some degree of planning".
The attack took place at an event on Wednesday for Farook's colleagues, which Farook attended.
He had been a San Bernardino public health employee for five years, police chief Jarrod Burguan said.
Mr Burguan said that Farook had left the event "under some circumstances that were described as angry".
He returned with Malik to open fire, armed with assault rifles and semi-automatic handguns and wearing military-style clothing.
Hundreds of people were evacuated as police officers swept through the massive complex.
Others locked themselves in rooms waiting for police to arrive.
Terry Petit's daughter, who works at the centre, sent him a text message from inside the centre, according to the Associated Press.
She wrote: "People shot. In the office waiting for cops. Pray for us. I am locked in an office."
Explosive devices found
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.
A third person seen running from the scene was detained, but police said it was unclear whether they were involved.
The authorities said all the weapons had been purchased legally but did not explain how they got into the hands of the perpetrators.
Three explosives devices were also found at the centre.
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.