A man and a woman suspected of taking part in a shooting that killed 14 people and wounded 17 at a Southern California social services agency on Wednesday died in a shootout with police hours later, authorities said.
A Muslim advocacy group said one of the suspects was Syed Farook, and introduced a man as Farook's brother-in-law, who said he had no idea what might have motivated the attack. He said his relative was a U.S. citizen.
Officials have yet to publicly identify any suspects.
The suspects fled the scene of the shooting in San Bernardino, about 60 miles (100 km) east of Los Angeles, and two people died a few hours later in a shootout when police confronted them in their getaway vehicle. One police officer was injured.
The shooting rampage at a holiday party on the campus of an agency that serves the developmentally disabled marked the deadliest U.S. gun violence since the massacre at Sandy Hook Elementary School in Newtown, Connecticut, in December 2012, in which 27 people, including the gunman, were killed.
At a news conference called by the Los Angeles area chapter of the Muslim advocacy group Council on American-Islamic Relations (CAIR), a man who identified himself as Farhan Khan said his sister was married to one of the suspects and he offered his condolences to the victims.
"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.
Farook and his wife have been missing since Wednesday morning, said Hussam Ayloush, executive director of CAIR in the Los Angeles area.
A person by the name of Syed Farook was listed on county documents as an employee of the San Bernardino County Environmental Health Department. Staff members from that department had gathered on Wednesday for the party where the shooters opened fire.
The massacre differed from most other recent U.S. killing sprees in key ways, including the involvement of multiple people rather than a lone perpetrator. It also comes less than three weeks after the deadly attacks in Paris prompted tighter security at many public venues across the United States.
Authorities said they also detained an individual seen running away from the vehicle, but investigators were not immediately sure that person was involved in the case, Police Chief Jarrod Burguan said at a news conference.
Burguan said the two suspects who were killed were armed with assault rifles and handguns and were dressed in "assault-style" clothing.
The police chief said he knew of no possible motive for the attack.
David Bowdich, an assistant regional FBI director, said authorities had not yet ruled out whether the shooting was an act of terrorism.