Mass shootings that involve multiple shooters are rare in American history: just two of the 28 deadliest shootings were carried out by more than one person.
The attack at the Inland Regional Center in San Bernardino, California, is now the third -- carried out by a man and a woman whose motives are still unclear.
Here is what we know about the attackers: Syed Rizwan Farook and Tashfeen Malik.
They are a couple
Police say Farook, 28, and Malik, 27, are either married or in a relationship. The Council on American Islamic Relations, a Muslim advocacy group, said Farook has been married for two years -- but wouldn't confirm whether Malik was the wife's name.
Farook and his wife left their 6-month-old girl with Farook's grandmother Wednesday and said they had a doctor's appointment.
The grandmother became concerned when she saw reports of the shooting and couldn't reach him.
Farook worked at the place he attacked
Farook, an American citizen, was an environmental health specialist with the San Bernardino County health department, which was hosting the holiday party at Inland Regional.
He had been there five years. Records list a man with his name and title who worked at the agency and made $53,000 in 2013.
He graduated from California State University San Bernardino with a degree in environmental health in 2009.
Police don't have a motive
Farook was at the party but left abruptly before the shooting. He seemed angry, witnesses told police. He returned. And, along with Malik, he went into the building and began firing.
There are no indications his job was in jeopardy, police said. They're also unaware of any criminal history.
Farhan Khan, Farook's brother-in-law, told reporters he too was at a loss.
"I have no idea why he would he do something like this. I have absolutely no idea. I am in shock myself," Khan said.
He said the family, like the rest of America, will have to wait for the police investigation for answers.
They didn't leave behind a note, a law enforcement official said.
He kept a low profile
Those who worked with Farook described him as quiet.
"I would say hi and bye. But we never engaged him in conversation. He didn't say much at all," said Griselda Reisinger, a former co-worker, to the Los Angeles Times.
Maria Gutierrez, who knew Farook, used the same word to describe him to The New York Daily News.
"He was quiet but always polite," she said.
He met his wife online
One co-worker, Patrick Baccari, told the Los Angeles Times that Farook recently visited Saudi Arabia to meet a woman he met online. They returned as husband and wife.
Farook had a profile set up on iMilap.com, which describes itself as "a site for people with disabilities and second marriage."
The profile said he "enjoys working on vintage and modern cars, reads religious books, enjoys eating out sometimes.
"Enjoys travelling and just hanging out in the back yard doing target practice with his younger sister and friends."
He said he was from a secular family
In the same profile, Farook said he came from a Muslim family with "Eastern and Western" family values.
There's no indication when the profile was set up. But his father told the New York Daily News Farook was very religious.
"He would go to work, come back, go to pray, come back," he said.
He and his brother share the same name
It's important to note Farook's middle name -- Rizwan. Because his older brother is also named Syed Farook but has a different s middle name.
Farook and Malik 'came prepared'
The attackers were "dressed in dark, kind of tactical gear," Burguan said. They carried assault rifles and semi-automatic handguns.
Two handguns recovered have been traced back to Farook. He purchased them legally three or four years ago, an official said. Two rifles were purchased by someone else, possibly a former roommate, also legally three or four years ago. (Officials don't think that person had anything to do with the shooting).
"These were people that came prepared," he said. "There had to have been some degree of planning that went into this."
They intended to inflict more damage
A bag believed to belong to the shooters was found inside the conference room. Inside, investigators found three three rudimentary explosive devices packed with black powder and rigged to a remote controlled car. The remote for the car was found inside the SUV where Farook and Malik were later killed, a law enforcement official said.
That means the pair planned to use the remote to detonate the explosives remotely, the official said. Either it didn't work because of distance or they didn't do it. Officials later rendered the explosive devices safe.
They died in a hail of bullets
After carrying out the attack at Inland Regional -- where they killed 14 people and injured at least 17 -- Farook and Malik led police on a chase. Farook fired at officers while Malik drove, officials said. They died in the shootout, which involved 21 officers.