REUTERS/Mike Blake
NBC News, CNN, The Associated Press, and The New York Times cited unnamed sources who said that Syed Rizwan Farook, a 28-year-old US citizen, seems to have been in touch with Islamist extremists at home and abroad.
Farook is suspected of carrying out the shooting at the Inland Regional Center - a county organization that provides services to the disabled - along with his wife, 27-year-old Tashfeen Malik.
Malik moved to California with Farook in 2014 and was living in the US under a K-1 visa with a Pakistani passport, said David Bowdich, assistant director of the FBI's Los Angeles bureau. Both suspects died Wednesday in a shootout with police.
CNN reported that Farook was in touch with extremists who were being investigated by the FBI for terrorism. The Times reported that the FBI is now treating the San Bernardino shooting as a counterterrorism investigation.
Law-enforcement officials told The Times that the FBI has evidence that Farook "was in contact over several years with extremists domestically and abroad, including at least one person in the United States who was investigated for suspected terrorism by federal authorities in recent years," according to the newspaper.
These communications were reportedly via phone and social media, according to CNN.
"He could have been radicalized, ready to go with some type of attack, and then had a dispute at work and decided to do something," a law-enforcement official told The Times.
Local police officials said at a press conference on Thursday that Farook was not on their radar for terrorism.
Authorities have not yet specified a motive for the attack.
Farook is thought to have traveled to Saudi Arabia in 2013 for the annual hajj pilgrimage to Mecca. He reportedly met Malik, a native of Pakistan, on that trip. He then reportedly traveled to Pakistan in 2014 and returned to the US with Malik.
Farook was born in Illinois to Pakistani parents, The Times reports. He worked at the San Bernardino County health department for five years. The shooting occurred during a holiday party for the agency.
Farook reportedly attended the party before he returned for the massacre, and left the event "under some circumstances that were described as angry," the San Bernardino police chief told reporters at a press conference. He came back with his wife to carry out the shooting at about 11 a.m. local time.
At Farook and Malik's home, authorities found "several hundred" .22-caliber long-rifle rounds, 12 "pipe bomb-type devices," "hundreds of tools, many of which could be used to construct IEDs or pipe bombs," 2,000 9 mm rounds, and 2,500 .223-caliber rounds of ammunition.
At the Inland Regional Center, the couple was reportedly armed with four guns: two semiautomatic handguns and two .223-caliber assault rifles, all of which were purchased legally. They also had 1,400 .223-caliber rounds and 200 9 mm rounds on them as they were pursued by police, officials said.