But it's unclear what drove the couple to storm a holiday party in San Bernardino and spray the husband's coworkers with bullets.
As more details come in about the attackers, so do the questions.
Was it a terror attack?
Shortly after the attacks, officials shied away from implying the massacre had any links to terror.
But the narrative changed Friday, when the FBI took over the investigation from San Bernardino authorities. The agency said it was treating the attack as an "act of terrorism."
There was "evidence ... of extreme planning" of the killings, said David Bowdich, an assistant FBI director.
And while the mass shooting may have been inspired by ISIS, a law enforcement official said, there's no indication the terror group directed or ordered the attack.
"This is looking more and more like self-radicalization," a law enforcement official said on condition of anonymity.
The couple did not have any trouble with the law, nor were they on any list of potentially radicalized people.
Relatives had no idea that the couple held radical views, according to family lawyers.
Was a religious dispute to blame?
Authorities have said in addition to terrorism, workplace disputes could be a possible motive. Or a combination of the two could have fueled the attack.
With every suggested motive are lingering questions.
If the attack was a result of workplace dispute, why did the killers have weapons in the home they lived in with their 6-month-old baby?
Shortly after the massacre, authorities said they searched their house and found pipe bombs, thousands of rounds of ammunition and more guns.
If it was terrorism, why did they choose to massacre his coworkers? Why did they pick a mundane, low-profile building that has no symbolic or historical significance?
Has ISIS claimed responsibility?
Yet the terror group has hailed the couple
, describing them as "supporters."
But for a group quick to claim credit and thump its chest after high-profile attacks, it was notable that ISIS did not say the couple were members or that it was responsible.
When claiming responsibility for other terrorist attacks, ISIS normally hails attackers as "knights" or "soldiers." This time, it stuck to "supporters," using its official radio station to say it hopes God will "accept them as martyrs."
"What they're calling these two are supporters, which is kind of a lesser level," indicating it might not have had direct contact with the couple, said Rick Francona, a CNN military analyst and a former intelligence officer.
That said, the group has made general, vague calls urging sympathizers to carry out attacks on their own.
Did the two have ties to ISIS?
Female shooter Tashfeen
Malik made a public declaration of loyalty to ISIS while the attack was underway. Three U.S. officials familiar with the investigation said she posted to Facebook a pledge of allegiance to ISIS leader Abu Bakr al-Baghdadi.
Facebook said it took the post down because it violated community standards that prohibit the promotion of terrorism or the glorification of violence. It declined to go into details about the nature of the post.
While the couple's motivation for the attack is a key focus for investigators, the terror group's acknowledgment of them as supporters doesn't mean they were members or that someone from the group ordered it, Francona said.
Officials briefing President Barack Obama about the investigation told him they had "no indication that the killers were part of an organized group or a broader terrorist cell," the White House said.
Did they travel to the Middle East?
Farook was born in Illinois to Pakistani parents and raised in California. He made two trips to Saudi Arabia.
The first visit was in 2013 for the Hajj, the annual pilgrimage to Mecca that Muslims are required to make at least once in their lifetimes.
The second time, he went to marry Malik, whom he'd met through an online dating service, family attorney Mohammad Abuershaid said.
The FBI said he went to Pakistan as well, but the family attorneys denied that.
, 29, was born and raised in Pakistan and moved to Saudi Arabia around age 20, the attorney said.
She came to the United States on a fiancée visa and became a legal permanent resident.
A federal official said Farook has "overseas communications and associations," but it's unclear how relevant they are to the shootings.
"We don't know yet what they mean," the official said.