Two heavily-armed individuals entered a social services center in San Bernardino Wednesday night, opening fire, killing 14 and wounding 17. A police manhunt ended in a standoff in which both attackers were killed.
Police identified the shooters as Syed Rizwan Farook, 28, and Tashfeen Malik, 27, described as a possibly married or engaged couple.
San Bernardino Police Chief Jarrod Burguan said Farook was a US-born county employee who had attended a holiday party at the Inland Regional Center, a social services agency, and later returned to open fire on the celebration.
The chief said Farook and Malik were believed to be the only shooters involved in the rampage, which ranks as the deadliest instance of US gun violence since the December 2012 massacre at Sandy Hook Elementary School in Newtown, Connecticut. While the motive remained unclear, Burguan said, "We have not ruled out terrorism." Farook was U.S. born, while Malik's nationality was still undetermined.
The suspect reportedly traveled to Saudi Arabia earlier this year and returned with a wife.
Co-worker Patrick Baccari says Syed Farook was gone for about a month in the spring. When he came back word got around Farook had been married, and the woman he described as a pharmacist joined him shortly afterward. A baby followed.
Baccari says the reserved Farook showed no signs of unusual behavior, although he grew out his beard several months ago.
Baccari said he been sitting at the same table as Farook at an office party Wednesday morning, but his co-worker suddenly disappeared, leaving his coat on his chair.
Baccari said he had stepped into the bathroom when the shooting started and suffered minor wounds from shrapnel slicing through the wall.
San Bernardino police Chief Jarrod Burguan said Farook and Malik were dressed in "assault-style" clothing during as they fled from police, and were armed with assault rifles and handguns.
At a news conference called by the Los Angeles area chapter of the Muslim advocacy group Council on American-Islamic Relations (CAIR), the brother-in-law of Farook, Farhan Khan, said he was bewildered by the news.
"Why would he do that? Why would he do something like this? I have absolutely no idea, I am in shock myself," Khan said at a news conference in Anaheim, California, south of Los Angeles.
Hussam Ayloush, executive director of CAIR in the Los Angeles area, said Farook and his wife had been missing since Wednesday morning.
Reuters and The Associated Press contributed to this report.