US President Barack Obama says investigators do not yet know why two suspects killed 14 people and wounded 17 others in a mass shooting in Southern California, but vowed that the FBI and law enforcement would “get to the bottom of this.”

“It is possible that this was terrorist-related,” Obama said.

“But we don’t know. It is also possible that this was workplace-related.”

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FBI’s assistant director in Los Angeles, David Bowdich, did not rule anything out – but said it would be “irresponsible” to call it terrorism for now.

“We cannot rule anything out at this point,” he said.

“There was obviously a mission here. We know that. We do not know why.

“It would be irresponsible of me to call this terrorism. The FBI defines terrorism very specifically. That is the big question for us, is what is the motivation for this.”

The San Bernardino shooting is the latest in a long series of US mass shootings during Obama’s seven years in office, and is the deadliest since the December 2012 shooting at Sandy Hook elementary school in Newtown, Connecticut, which he has said was his toughest day as president.

Speaking from the Oval Office after meeting with his national security advisers on Thursday, Obama expressed sympathy for the victims and said the United States needs to pass laws to “make it a little harder” for people seeking to do harm to get access to weapons.

But Obama took a less angry tone than he has used after other recent mass shootings, and sought to reassure Americans who are nervous after attacks in Paris by Islamic State militants last month.

He said the Federal Bureau of Investigation would take the lead in the investigation and would do a “large number of interviews” and pore over “social media and electronic information.”

But Obama said the shooters may have had “mixed motives” which could make the investigation more complicated, and warned it could take “some time” to reach conclusions.

“We do know that two individuals who were killed were equipped with weapons and appeared to have access to additional weaponry in their home. But we don’t know why they did it,” he said.

“We don’t know at this point the extent of their plans. We do not know their motivations,” he said.

A couple embraces following the shooting that killed multiple people. Photo: AAP

Officials in Washington familiar with the investigation said so far there was no hard evidence of a direct connection between the shooters and any militant group abroad.

The sources said authorities had raided a townhouse believed used by the couple in search of electronic devices that could show if they had been browsing on Jihadist websites or social media.

CNN, citing law enforcement sources, said Farook had been “radicalised.”

CNN also said he had been in touch through telephone and social media with more than one international terrorism suspect who was being investigated by the FBI.

Farook, a US citizen, was born in Illinois, the son of Pakistani immigrants, according to Hussam Ayloush, who heads the Los Angeles area chapter of the Muslim advocacy group Council on American-Islamic Relations (CAIR).

Malik, his wife, was born in Pakistan and had been living in Saudi Arabia before marrying Farook, Ayloush said.