Enrique Marquez is accused of conspiring to commit terrorism in 2011 and 2012
A friend of the married couple behind this month's massacre in San Bernardino, California, has been arrested on terrorism charges in connection with the attackers.
Enrique Marquez, 24, is accused of conspiring with Syed Rizwan Farook to commit terrorism in 2011 and 2012.
He is also charged with illegally purchasing the two AR-15 assault-style rifles that Farook and his wife Tashfeen Malik used to kill 14 of Farook's colleagues at an office party on 2 December.
Authorities previously said Marquez had legally purchased the high-powered guns.
The Muslim convert is further charged with defrauding US immigration authorities by entering into sham marriage with a relative of one of the attackers.
Investigators had been looking into Marquez's 2014 marriage to a Russian woman whose sister wed Farook's older brother.
Marquez and his wife apparently do not live together.
Marquez - a former neighbour of 28-year-old Farook - checked himself into a Los Angeles-area psychiatric facility after the shootings.
The former Walmart security guard posted a garbled Facebook message that read: "I'm. Very sorry sguys. It was a pleasure," reports the Los Angeles Times.
His arrest comes a day after Farook and 29-year-old Malik were buried in a Muslim ceremony, guarded by the FBI.
The couple's funeral took place at a graveyard in southern California, hours from the scene of the attack that killed 14 people.
Two mosque members told Reuters news agency many fellow members of the attackers' mosque had shunned the service.
About 10 people did attend the funeral, including fellow mosque-goers and members of Farook's family.
Muslims are usually buried within 24 hours of dying, but relatives and community members had to wait for the bodies to be released by law enforcement officials.
It took another week for the funeral organisers to find a graveyard willing to accept the bodies.
Farook and Malik left behind a six-month-old daughter, who has been in state custody since the massacre.
Farook's sister and brother-in-law have said they hope to adopt their niece.
President Barack Obama is due to travel to San Bernardino on Friday to meet privately with victims' families.