A friend of one of the San Bernardino attackers has been arrested in connection with the mass shooting that killed 14, US media report.
Enrique Marquez is the first arrest tied to the deadliest terrorist attack in the US since 9/11.
He bought two semiautomatic rifles that were used by attackers Syed Farook and Tashfeen Malik, who died during a shoot out with police following their rampage at a health centre.
Mr Marquez, 24, told authorities Farook, his former neighbour, asked him to buy the rifles in 2011 and 2012.
No formal charges have been brought against him yet.
He is being held without bail at a detention centre in Riverside, California, the Washington Post reports, with his first hearing scheduled for 21 January.
Federal authorities have said he is cooperating with the investigation.
Mr Marquez, a convert to Islam, is a relative of Farook by marriage. Last year, he married Tashfeen Malik's sister.
Federal agents raided his mother's home, where he had been living, three days after the attack.
Investigators are looking into whether he and Farook were planning an attack in 2012 but decided not to follow through because of terrorism-related arrests in the area.