(CNN)They were a married couple who left their baby with grandma while they carried out the deadliest mass shooting in the United States since the December 2012 rampage at Sandy Hook.
Who were Syed Rizwan Farook and Tashfeen Malik?
Farook and Malik were later killed in a shootout with police. The attackers' specific motives remain unclear, but here is what we know about them:
Farook, 28, and Malik, 29, were married.
He got to know Malik through an online dating service, Farook family attorney Mohammad Abuershaid said. They met face to face when Farook visited Saudi Arabia, where Malik had moved from her native Pakistan around the age of 18 or 20, according to the lawyer.
She later came to the United States on a fiancée visa and became a legal permanent resident.
The couple left their 6-month-old girl with Farook's mother Wednesday, saying they had a doctor's appointment.
The grandmother became concerned when she saw reports of the shooting and couldn't reach Farook.
Farook, an American citizen, was an environmental health specialist with the San Bernardino County health department, which was hosting the holiday party at the Inland Regional Center where the attack took place.
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 2010.
A graduate student ID card from California State University, Fullerton, with Farook's name on it was spotted by CNN on Friday inside his Redlands townhouse. Jeff Cook, a spokesman for that school, said that records indicate a student by that same name had been enrolled in Cal State Fullerton's environmental engineering graduate program.
The university issued a statement Friday saying, "Syed Rizwan Farook attended Cal State Fullerton for one semester in Fall 2014 in the online graduate program in environmental engineering. He is not currently enrolled."
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, authorities said.
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 was at a loss, too.
"I have no idea why he would 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.
Farook and Malik didn't appear to have left behind a note, a law enforcement official said.
Law enforcement officials told CNN that Farook had been in touch via phone and social media with at least one person whom the FBI suspected of international terrorism. Officials wouldn't elaborate.
It appears Farook was radicalized, which contributed to his motive, though other things -- such as workplace grievances -- may have also played a role, other law enforcement sources said.
The officials said these contacts were not considered high-priority terrorism subjects. And it had been at least a few months since the last known communication, they said.
Neither Farook nor Malik were on any list of potentially radicalized people, and there are no clear ties to overseas terrorist groups as of now, officials said.
Farook's relatives knew him as conservative but didn't think he or his wife were extreme, according to the family's attorneys.
"He was a normal guy," Abuershaid said.
As the attack was underway, investigators believe Malik expressed allegiance to ISIS leader Abu Bakr al-Baghdadi in a Facebook post, three U.S. officials familiar with the investigation told CNN.
Malik made the post on an account with a different name, one U.S. official said. The officials did not explain how they knew Malik was behind the message.
A law enforcement official said it appeared ISIS may have inspired the attack, but none of the officials said the terror group directed or ordered the attack.
"This is looking more and more like self-radicalization," one law enforcement official said.
Those who knew Farook described him as quiet.
"He's a little bit shy, a little bit withdrawn. He doesn't mix with people easily," said Mustafa Kuko, director of the Islamic Center of Riverside where Farook was a regular.
Doyle Miller, Farook's landlord, said he had "no cause for concern" when he rented out a townhouse to him in Redlands.
"He had no red flags whatsoever; everything checked out," Miller said. "He had good credit reports ... everything.
"We screen our people pretty good. It's unbelievable what's going on."
Farook's family knew him as someone who "always kept to himself," as did his wife, according to Abuershaid.
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."
It also said Farook "enjoys traveling and just hanging out in the backyard doing target practice with his younger sister and friends."
He would go to firing ranges by himself, said David S. Chesley, another lawyer for Farook's family.
A representative for iMilap told CNN Farook logged on just twice in 2010 and wasn't active after that. He didn't contact anyone, nor did anyone contact him, the site said.
His father told the New York Daily News that Farook was religious.
"He would go to work, come back, go to pray, come back," he said.
Kuko, the Islamic Center director, said Farook would come to the center for morning and night prayers.
It's important to note Farook's middle name -- Rizwan. His father also is named Syed Farook, as is his older brother. Like him, they have different middle names.
San Bernardino: Full coverage
The couple was "dressed in dark, kind of tactical gear," San Bernardino police Chief Jarrod Burguan said. They carried rifles and semiautomatic handguns.
Two handguns recovered have been traced 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 said they don't think that person had anything to do with the shooting.
"These were people that came prepared," Burguan said. "There had to have been some degree of planning that went into this."
Two relatively new cellphones were found smashed and tossed in a garbage can near one of the crime scenes, law enforcement officials said. Officials suspect the phones were damaged to hide call histories.
A computer found at the couple's home was missing a hard drive. Investigators have subpoenaed email service providers to retrieve communications.
Inside the garage of the couple's rented townhouse, authorities found 12 pipe bomb-type devices and hundreds of tools, many of which could be used to make improvised explosives.
Authorities also found thousands of round of ammunition.
A bag believed to belong to the shooters was found inside Inland Regional's 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 car was found inside the SUV where Farook and Malik were later killed, a law enforcement official said.
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 didn't do it. Officials later rendered the explosive devices safe.
After carrying out the attack at Inland Regional, 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.