Enrique Marquez, 24, is charged with conspiring with Farook to provide material support to terrorism, making a false statement in connection with the acquisition of firearms, and immigration fraud, according to a statement from the Department of Justice.
He is believed by authorities to have bought two assault rifles used by Farook and Farook's wife, Tashfeen Malik, in the December 2 mass shooting and terror attack that left 14 people dead in the Southern California city.
Marquez made an initial appearance in court on Thursday afternoon. He wore handcuffs and eyeglasses, and swiveled in his chair a couple of times before the proceedings began. His lawyer, Young Kim, declined to comment on the case as he left court.
Marquez has told investigators he didn't know about the plans for the San Bernardino attack, U.S. officials have said, and the complaint against him does not allege that Marquez was involved.
It does allege, however, that Marquez bought the AR-15-style rifles used in the shooting, as well as explosive material used to construct a pipe bomb that was found at the attack site. The complaint also accuses Marquez of planning terrorist attacks with Farook in 2011 and 2012 that were not carried out, the Justice Department said.
Marquez reportedly told investigators he and Farook had planned to attack the library or cafeteria at Riverside Community College. They also, allegedly, had plotted to attack State Route 91 during rush hour. Marquez told investigators that the plan was to throw pipe pipe bombs on the freeway, and then to open fire on stopped vehicles, the Justice Department said.
Neither attack took place. After 2012, Marquez purportedly stopped planning with Farook and distanced himself from his friend after the arrests of Ralph Deleon and others
on terrorism-related charges.
Additionally, Marquez is accused of entering a sham marriage with a member of Farook's family, according to the complaint.
"Mr. Marquez conspired with Mr. Farook to commit vicious attacks, as set forth in today's charges," said U.S. Attorney Eileen M. Decker.
"Even though these plans were not carried out, Mr. Marquez's criminal conduct deeply affected San Bernardino County, Southern California and the entire United States when the guns purchased by Marquez were used to kill 14 innocent people and wound many others."
911 call: 'They can trace all the guns back to me'
Since the San Bernardino shootings, Marquez has waived his Miranda rights, cooperated with investigators and provided information, officials have said. He also checked himself into a mental health facility.
According to the complaint, Marquez called 911 on December 3. He told the operator that Farook was behind the San Bernardino attack.
"The f****** a****** used my gun in the shooting," Marquez said. "They can trace all the guns back to me."
When asked how Farook got his gun, Marquez said that he'd given it to him for safe storage, the complaint read.
"To me, he was reliable enough for him for storage, like to store my gun," Marquez allegedly told the operator.
Farook and Malik opened fire on Farook's co-workers at a work event December 2. Police shot and killed them hours later.
The massacre was a terror attack, authorities have said. Both U.S.-born Farook and Pakistan native Malik expressed commitment to Islamic jihad and martyrdom in direct, private online messages in late 2013, months before Malik moved to the United States to live with Farook, FBI Director James Comey has said.
Malik also pledged allegiance to ISIS leader Abu Bakr al-Badghadi in a Facebook post as the San Bernardino attack was happening, three U.S. officials familiar with the investigation told CNN.
Authorities still are trying to learn more about whom the killers interacted with, how they hatched and carried out the plot and why.
Neighbors, rifles and pipe bombs
Marquez and Farook were neighbors in Riverside, California, until Farook and Malik moved recently to nearby Redlands. Investigators have been delving into Marquez's relationship with Farook.
Marquez was a state-licensed security guard until his license expired last year. At one point he converted to Islam and attended the same mosque as other members of the Farook family, though a mosque facility manager told CNN that Marquez stopped attending the mosque about two years ago.
Marquez bought one rifle in 2011 and another later, and gave both to Farook but did not report the transfer of ownership, two law enforcement officials told CNN on condition of anonymity.
He also told investigators that he used to build pipe bombs with Farook, portraying himself and his friend as hobbyists experimenting with building the devices, law enforcement officials said.
Marquez said he had nothing to do with the pipe bombs that authorities found at Farook's home after the December 2 attack, or the ones that apparently failed to go off at the site of the shooting, the officials said.
Talking to CNN last week, neighbors of the Riverside homes where Marquez and Farook lived next door to one another recalled the two working on cars together but did not know their relationship extended beyond that shared interest.
As for the killers, the bodies of Farook and Malik were buried Tuesday, 13 days after the attacks, according to the Law Offices of David S. Chesley, the California attorney who represents Farook's family. A funeral for the couple Tuesday in Rosamond, north of Los Angeles, was attended by family members and "very few" friends, a source told CNN on condition of anonymity.
President Barack Obama is expected to travel to San Bernardino on Friday
to meet privately with families of the victims of this month's attack.