Authorities are working to determine why a man and a woman opened fire at a holiday party of his co-workers in Southern California yesterday, killing 14 people and wounding 21 in what appeared to be a planned attack.

Syed Rizwan Farook, 28, and Tashfeen Malik, 27, who had a six-month-old daughter together, were killed in a shootout with police after the mass shooting at the Inland Regional Centre in the city of San Bernardino, a social services agency where Farook worked as an inspector.

Officials from San Bernardino Police Chief Jarrod Burguan to President Barack Obama said the attack may have been an act of terrorism but that a motive had not yet been determined.

“It is possible that this was terrorist-related. But we don’t know,” Obama told reporters at the White House. “It is also possible that this was workplace-related.”

“Rest assured that we will get to the bottom of this,” said Obama, who said the FBI was taking over the investigation of the attack.

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 Syed, Ayloush said. Both were Muslims, Ayloush said.

Officials in Washington familiar with the investigation said so far there was no hard evidence of a direct connection between the shooters and any foreign militant group. The sources said a raid on a townhouse believed used by the couple was searched for electronic devices that could show if they had shown interest in Jihadist websites or social media.

The two assault rifles and two handguns recovered from the shootout were legally purchased in the United States, said a spokeswoman for the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). Two of them were purchased by someone “associated with this investigation,” while the buyer of the other two was not linked to the investigation, she said.

The long guns were .223-caliber and their ammunition can go through protective vests and walls, Davis said.