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 Newtown, Connecticut, in December 2012, where 26 people, mostly children, died.
With the investigation still unfolding, much is unclear about Wednesday's attack at a holiday party at the center for people with developmental disabilities.
We know there were two shooters: A U.S. citizen named Syed Rizwan Farook, 28, who was an inspector with the county health department that was hosting the party, and his wife, 27-year-old Tashfeen Malik. Law enforcement sources say Farook had been in contact with more than one person who had been the subject of an international terrorism investigation by the FBI.
For now, here is what we know and don't know about the attack and its aftermath:
The attack: The attack took place around 11 a.m. 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 came back with Malik. They were armed and fired 65 to 75 rounds, police said, killing at least 14 people and wounding at least 21.
The couple then escaped in a dark-colored, rented Ford SUV with Utah plates.
The police response: Authorities -- acting on a lead about where Farook lived -- went to a home in Redlands, about 10 miles from San Bernardino, to serve a search warrant.
While they were 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: The pursuit ended back in San Bernardino, about 2 miles from Inland Regional. A shootout ensued, with Farook firing at officers from the vehicle while Malik drove, officials said.
Farook and Malik were killed. Twenty-three officers fired about 380 rounds at the couple, who fired back with 76 rounds.
The shooters had on their person or in their vehicle more than 1,400 rounds of .223-caliber ammunition and more than 200 9mm rounds, Burguan said.
The raid: Law enforcement personnel moved in on the home in Redlands. They used a robot to sweep for explosives. They asked "immediately adjacent" neighbors to evacuate until they were sure the place was safe.
In the home and garage, police found 12 pipe bomb-type devices, plus hundreds of tools, many of which could be used to construct improvised explosive devices, or pipe bombs, Burguan said.
Police also found another 2,000 9mm rounds, 2,500 .223 rifle rounds and several hundred more rifle rounds.
The attackers wore tactical gear that allowed them to carry items such as ammunition, but they weren't wearing protective armor, Burguan said. They carried rifles and semi-automatic handguns.
Two of the firearms have been traced back to them; they were purchased legally. Two rifles were purchased -- also legally -- by someone else, possibly a roommate, an official said. Authorities don't think that person had anything to do with the attack.
"These were people that came prepared," Burguan said. "There had to have been some degree of planning that went into this."
And they apparently intended to inflict more damage.
A bag believed to belong to the shooters was found in the conference room where the party was held. Inside, investigators found three rudimentary explosive devices packed with black powder and rigged to a remote-controlled toy car. The remote for the toy car was found inside the SUV, a law enforcement official said.
That means the pair planned to use the remote to detonate the explosives from a distance, the official said. Either it didn't work because of distance or they just didn't do it. Authorities later rendered the explosive devices safe.
Farook 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," Khan said, expressing condolences for the victims and their families. "I am in shock myself," Khan said.
On Thursday, law enforcement officials said that Farook was in touch over the phone and via social media with more than one person who had been the subject of an FBI international terrorism investigation. It appears that he was radicalized, which contributed to his motive, though other things -- like workplace grievances -- may have also played a role, other law enforcement sources said.
Hussam Ayloush, head of the Council on American-Islamic Relations' Los Angeles area chapter, said Farook and Malik had been married for two years.
They left their 6-month-old daughter 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.
Neither Farook nor Malik was previously known to the FBI, law enforcement officials said. Nor were they on a list of potentially radicalized people. And police did not know of any interactions they'd had with officers prior to Wednesday.
Authorities are also working to determine whether the couple acted alone or had help, either during the attack itself or before.
Police did detain one additional person who was seen running away after the shootout. But they don't know whether he was involved in the attack.
At least 14 people were killed. At least 21 others were injured -- many by bullets, others in the panic to escape.
A San Bernardino police officer was shot in the leg during the gunbattle but was expected to be released from the hospital on Thursday, police said. A sheriff's deputy suffered cuts on the leg, perhaps from shrapnel.
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 escorted people out of their offices.
Arrowhead Regional Medical Center and Loma Linda University Medical Center said Thursday, on their respective Twitter sites, that each had five patients.
The latter hospital's CEO, Kerry Heinrich, said all the victims treated there had gunshot wounds and two of them were in critical condition. As they fought to survive, Heinrich noted, they and hospital staff also had to deal with "officers with assault rifles going unit to unit looking for devices."
"It is something that our staff understands," he said. "But it is never easy to go through."
The county health department was hosting a holiday party at the center. 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 adds complications officials still can't explain.
The shooters didn't appear to have left a note, a law enforcement official said.