Authorities sought on Thursday to determine if a couple accused of killing 14 people in a mass shooting at a workplace holiday party in Southern California had links to Islamic militant groups abroad, U.S. officials familiar with the investigation said.

Syed Rizwan Farook, 28, and Tashfeen Malik, 27, who had a 6-month-old daughter together, were killed in a shootout with police after Wednesday's massacre at the Inland Regional Center social services agency in the city of San Bernardino.

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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.

Police said a raid of a townhouse believed used by the couple yielded flash drives, computers and cellphones that would be checked to see if the couple had been browsing on Jihadist websites or social media.

"Rest assured that we will get to the bottom of this," President Barack Obama said at the White House, adding that the FBI was taking over the investigation.

Farook and Malik were identified as Muslims and David Bowdich, the FBI assistant director in Los Angeles, said the couple had returned to the United States in July 2014 after a trip that included Pakistan and perhaps other countries. Farook did not have a criminal record, police said.

CNN, citing law enforcement sources, said Farook had been "radicalized." 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.

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Officials from Obama to San Bernardino Police Chief Jarrod Burguan 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. "It is also possible that this was workplace-related."

Farook, a U.S. 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 was born in Pakistan and had been living in Saudi Arabia before marrying Farook, Ayloush said.

Burguan said at a news conference on Thursday that 21 people were wounded, four more than previously stated. Ten people remained hospitalized at two hospitals on Thursday - two in critical but stable condition, three in fair condition and five in stable condition, the hospitals said.