U.S. prosecutors are considering filing firearms charges against Marquez, with state gun charges also possible.
Enrique Marquez, a friend and former neighbor of radicalized Muslim Syed Rizwan Farook, 28, who carried out the Dec. 2 attack with his wife, Tashfeen Malik, 29, could be charged as early as Thursday, CNN and NBC reported, citing unnamed law enforcement officials.
U.S. prosecutors are considering filing firearms charges against Marquez, with state gun charges also possible, one of the government sources told Reuters on condition of anonymity.
Marquez, 24, who had checked himself into a Los Angeles-area psychiatric facility shortly after the shootings, had several connections to Farook and Malik and quickly became a key figure in the investigation of the shootings.
The Federal Bureau of Investigation, which is treating the attack as an "act of terror," raided his home and questioned him for several days. Sources said Marquez cooperated during their interviews.
During the investigation, a law enforcement source said Marquez, who had converted to Islam, and Farook apparently had plotted some sort of attack around 2012 but abandoned the idea.
Marquez, who had known Farook since they were teenage neighbors in the city of Riverside, legally purchased the two AR-15 assault-style rifles that the couple used in their attack on a holiday party of Farook's co-workers. Farook, the U.S-born son of Pakistani immigrants, and Pakistani-born Malik were killed in a shootout with police a few hours after their assault on the party.
Marquez, who had worked at Walmart and at a bar recently, also is related to Farook's family by marriage. In 2014, according to state records, Marquez married a Russian woman who was the sister of Farook’s older brother’s wife. Neighbors said they were surprised to learn that he had been married, having never seen him with a woman.
The FBI said Farook and Malik had discussed martyrdom online before they even met and were supporters of Islamic State, the violent group that has taken over large parts of Syria and Iraq.
Their attack, which left 21 people wounded, has stirred concerns among Americans about national security and the reach of Islamic State, becoming an issue in the U.S. presidential campaign. The attack came a few weeks after gunmen and suicide bombers affiliated with Islamic State killed 130 people in a series of coordinated attacks in Paris.
FBI Director James Comey on Wednesday said there was no evidence that the San Bernardino attackers, who were also inspired by Islamic State, had been part of a terrorist cell.
Neighbors said Marquez and Farook often worked together on cars in Farook's garage in their younger years but that the friendship had cooled in the past three years.
Marquez converted to Islam about the time that Farook became more devoted to the faith around 2008. But Azmi Hasan, manager at the Islamic Society of Corona-Norco, said Marquez later told him that Islam was not for him.
President Barack Obama is due to travel to San Bernardino on Friday to meet privately with families of the San Bernardino victims.