(CNN) — 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 the attack 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 holiday party, and his wife, 27-year-old Tashfeen Malik.

But we don’t know their 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. 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. He and Malik went into the building and began firing, police said. At least 14 people were killed and at least 17 were injured, police said.

The couple then escaped in a dark-colored SUV.

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: A shootout ensued, with Farook firing at officers from the vehicle while Malik drove, officials said.

The pursuit ended back in San Bernardino, about 2 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 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.

THE PLOT
The attackers were “dressed in dark, kind of tactical gear,” 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. 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.

THE ATTACKERS
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.

Hussam Ayloush, head of the Council on American Islamic Relations’ Southern California 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.

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 escorted 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 adds complications officials still can’t explain.

The shooters didn’t appear to have left a note, a law enforcement official said.

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