Since then, federal authorities have said the couple was "self-radicalized" in part by reading radical Islamic literature online.
But why did they pick the Christmas gathering held by colleagues of Farook, who worked as a health inspector for San Bernardino County?
Here is we know:
Why was the IRC targeted?
Officials said they don't know for sure.
San Bernardino Police Lt. Mike Madden noted that in 2014, Farook attended another work event in that room, which was decorated for Christmas. Malik later stated in an online account that she felt Muslims should not have to attend Christian events, Madden said.
Madden added that she did not specifically mention the event in her posting and that it's far from clear that it played any role in last year's attack.
"It is open to speculation why they chose the party," he said. "I don't know if we can say with any certainty why the party was chosen apart from it was a soft target."
San Bernardino Police Chief Jarrod Burguan agreed.
"There are a lot of questions, and we don't have all the answers," he said Thursday. "We never established the motive. The best we can do is theorize."
What was the timeline of events?
Farook left his Redlands home at 8:37 a.m. Dec. 2 and arrived at the Inland Regional Center 10 minutes later. His co-workers were gathered for a holiday event and training session there. He carried with him a bag containing a bomb, authorities say.
He stuck around the center for about two hours before leaving. He returned to the event at 10:56 a.m. with his wife. They were armed with AR-15s, and their faces were covered. They sprayed the room with bullets before fleeing. They left behind an explosive device made of several pipe bombs.
They later were killed by police at the end of a high-speed chase.
How extensively did they plan an attack?
Officials believe they planned for months, gathering weapons and building bombs in their garage.
Farook traveled to the Middle East before the attack and came back with his wife. Farook was born in Illinois, but Malik was raised in Pakistan and lived in Saudi Arabia before marrying him. From the beginning, one of the biggest questions was whether the attack was part of a larger international plot hatched by Islamic State or some other Islamic terror group. Malik posted a note on Facebook pledging allegiance to Islamic State shortly after the attack, authorities said.
The FBI concluded fairly quickly that the two were "self-radicalized." They were inspired by terrorist groups, officials said, but did not receive financial support from any foreign or domestic organizations.
A year later, the investigation continues.
What are memorial plans today?
San Bernardino will sponsor a "Night of Remembrance" Friday at 6:30 p.m. at Cal State San Bernardino's Coussoulis Arena.
"Our community continues to stand together united in solidarity and support for the victims, survivors, and families of those affected by the December 2nd terrorist attack," Mayor R. Carey Davis said in a statement.
A memorial bicycle ride is planned to the IRC Friday morning.
A special lighting formation in the hills above San Bernardino to memorialize the victims is also planned.
7:55 a.m.: This article was updated with memorial information.