The San Bernardino shooters were a married couple who left their six-month-old daughter with family hours before the rampage, saying they were going to a doctor's appointment.
Fourteen people were left dead and 21 were wounded before the duo was killed during a shootout with police in a residential neighborhood a mile away. Authorities revealed on Thursday that the couple had more than 1600 rounds of ammo in their possession.
Details are now beginning to emerge about the killers.
Farook was born in Illinois and raised in Southern California, although his family is originally from Pakistan, according to Hussam Ayloush, executive director of the Council on American-Islamic Relations (CAIR).
Farook traveled to Saudi Arabia for about a month in the spring and returned with a wife, his co-worker Patrick Baccari told the Associated Press. Ayloush told Reuters that the couple had been married for two years.
The couple had a baby girl, who is now six months old.
Farook worked as an environmental health specialist for San Bernardino County, according to authorities. His job, which had a salary of $51,000, involved inspecting restaurants for health violations and inspecting public pools at apartments and senior housing complexes.
Colleagues described him as reserved, and said he showed no signs of strange behavior after returning from his trip earlier this year. Baccari, who shared a cubicle with Farook, said the only difference following the trip was that he grew out his beard.
Other colleagues told the Los Angeles Times that Farook was a devout Muslim.
"He never struck me as a fanatic, he never struck me as suspicious," Griselda Reisinger said. "He didn’t say much at all."
A profile in his name on a dating site says he's from a religious "but modern family." The profile was apparently written when he was 22.
He writes that he enjoys "working on vintage and modern cars, read religios [sic] books, enjoy eating out sometimes travel and just hang out in back yard doing target pratice [sic] with younger sister and friends."
Farook is pictured in the profile, seen below:
On another dating website, Arab Lounge, he wrote: "I try to live as a good Muslim, looking for a girl who has the same outlook, wear hijab."
He added that he wanted to meet a girl "for snowboarding" and "working on cars with me."
On Wednesday morning, the couple left their daughter with Farook's mother, saying they were taking Malik to a doctor's appointment, according to Ayloush.
He later attended his department's annual holiday party. Baccari said he had been sitting at the same table as Farook when he suddenly disappeared, leaving his coat behind. Farook returned with his wife and weapons and opened fire.
San Bernardino Police Chief Jarrod Burguan said the shooting spree had been planned beforehand and that the couple left pipe bombs behind before fleeing in a dark SUV.
Farhan Kahn, who is married to Farook's sister, said at a CAIR press conference on Wednesday that he was shocked to hear his brother-in-law was involved in the shooting.
"I have no idea why he would do that," he said. "I am in shock that something like this could happen. I am very sad that people lost their lives."
At a press conference late Thursday, San Bernardino Police Lt. Mike Madden described what it was like to be the first on scene.
"The situation was surreal," he said. "It was unspeakable the carnage we were seeing ... .The pure panic on the faces of those individuals that were still in need and needing to be safe."
The noise was cacophonous, he said, with victims moaning and wailing and the building's fire alarm going off.
In fact, the family thought Farook could be a victim when they heard about the shooting, Ayloush said.
When the couple didn't return to collect their daughter hours after dropping her off, his family started to worry, especially when they saw reports of the incident. The family only started to realize what had happened when they were contacted by a reporter, Ayloush said.
"We don't know the motives," Ayloush said. "Is it work, race-related, is it mental illness, is it extreme ideology? At this point, it's really unknown to us and at this point it's too soon to speculate."