Enrique Marquez, 24, is detained on counts of supporting terrorism, but is not believed to have had advance knowledge of the attack.
A first arrest has been made in connection with the mass shooting in San Bernardino that killed fourteen people.
Enrique Marquez, 24, has been charged on three counts, including plotting a university attack with gunman Syed Farook in 2011 and 2012.
— KTLA (@KTLA) December 18, 2015
He is alleged to have been a friend of Farook and his wife and co-conspirator Tashfeen Malik, and was once Farook’s next-door neighbour.
Marquez is also accused of illegally buying both rifles used in the early December shooting spree.
Both Farook and Malik were shot dead by police a matter of hours after carrying out the California attack.
Los Angeles Attorney Eileen Decker says there is currently no evidence Marquez had prior knowledge the shooting would take place, nor that he participated in it.
“While there is currently no evidence that Mr Marquez participated in the December 2 attack or had advance knowledge of it, his prior purchase of firearms and ongoing failure to warn authorities about Farook’s intent to commit mass murder had fatal consequences.”
Hours after the rampage, he reportedly called emergency services to say the attackers had “used my gun.” It is the deadliest attack on US soil since 9/11.
— Joshua Saul (@joshfromalaska) December 17, 2015
Shortly after the attack, Marquez checked himself into a mental health facility.
The FBI, which is treating the rampage as terrorism, says Marquez has cooperated during interviews.