The shooting at the Inland Regional Center in San Bernardino is the deadliest mass shooting in the United States since the shooting at the Sandy Hook Elementary School in Newton, Connecticut, in December 2012, where 26 people, mostly children, died.
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With the investigation still unfolding, much is unclear about the attack at the center for people with developmental disabilities.
We know the shooter was a U.S. citizen named Syed Rizwan Farook, 28, an inspector with the county health department that was hosting the holiday party. He was later killed in a shootout with police, along with a woman -- Tashfeen Malik, 27 -- who is either his wife or his girlfriend.
But we don't know Farook's motives. We also don't know the identities of the victims, nor do we have a full accounting of how many were injured.
For now, here is what we know and don't know about the attack and its aftermath:
THE TIMELINE
The attack: The attack took place around 11 a.m. Pacific time (2 p.m. ET) during a holiday party at the Inland Regional Center in San Bernardino, California. It's a facility for people with developmental disabilities.
Farook abruptly left the center before the shooting "under circumstances described as angry," said San Bernardino Police Chief Jarrod Burguan.
Soon after, Farook walked in and began firing. At least 14 people were killed and at least 17 were injured, police said.
He then escaped in a dark-colored SUV.
The police response: Athorities -- acting on a lead where Farook lived -- went to a home in Redlands, about 10 miles from San Bernardino, to serve a search warrant.
While there, a black SUV with Utah plates passed by slowly, then sped up and raced off, a law enforcement official close to the investigation said.
A police cruiser pursued it.
The shootout: A shootout ensued, with Farook firing at officers from the vehicle while Malik drove, officials said.
The pursuit ended back in San Bernardino, about two miles from Inland Regional.
Farook and Malik were killed in a shootout with police that involved 21 officers, Burguan said.
The raid: Law enforcement personnel have moved in on the home in Redlands. They were using a robot to sweep for explosives. They've asked "immediately adjacent" neighbors to evacuate until they're sure the place is safe.
THE SUSPECTS
There were two -- Farook and Malik.
The attackers were "dressed in dark, kind of tactical gear," Burguan said. They were assault rifles and semi-automatic handguns.
Two of the firearms have been traced back them; two others to another person that officials want to talk but who isn't believed to have been involved, a law enforcement official said.
"These were people that came prepared," he said. "There had to have been some degree of planning that went into this."
Farook, who's been identified as one of the suspects, is an environmental health specialist for San Bernardino County. He had worked there for five years.
Farhan Khan, Farook's brother-in-law, told reporters Wednesday night he last talked to Farook a week ago.
"I have no idea why he would he do something like this. I have absolutely no idea. I am in shock myself," Khan said.
Hussam Ayloush with the Council on American Islamic Relations said Farook was married for two years. He wouldn't confirm whether Malik was the wife.
Farook and his wife left their 6-month-old girl with Farook's grandmother Wednesday and said they had a doctor's appointment, Ayloush said.
The grandmother became concerned when she saw reports of a shooting at Farook's workplace. She called him but never got a response.
Police detained another person who was seen running away after the shootout. But they don't know whether he was involved in the attack.
THE VICTIMS
At least 14 people were killed. At least 17 others were injured -- many by bullets, others in the panic to escape.
The shootings took place in the conference center at Inland Regional. It's unclear how many people were at the facility at the time of the shooting.
Most of the victims were "centrally located in one area of the facility," said Burguan.
Lavinia Johnson, the center's executive director, said the fire alarm went off in her building, and people began to evacuate but then the order came to stay in place. Later police came and took people out of their offices.
THE MOTIVE
Unclear.
The center was hosting a holiday party. Farook was at the party, just as he'd been last year, police said. He abruptly left.
So was a dispute at the party at the root of the attack?
There are no indications his job was in jeopardy, Burguan said. He also was unaware of any criminal history.
A law enforcement official said the scale of the attack and the fact there was others involved adds complications officials still can't explain.
THE CENTER
The center employs nearly 670 people at its facilities in San Bernardino and Riverside counties, providing service to more than 30,200 people. It offers one-on-one care to people with developmental disabilities. San Bernardino is a city of just over 200,000 people about 60 miles east of Los Angeles.
