Federal authorities are preparing criminal charges against a man who investigators say supplied guns to the married couple who killed 14 people at a holiday party in San Bernardino, California, this month, two government sources said on Thursday.

Enrique Marquez, a friend and former neighbor of radicalized Muslim Syed Rizwan Farook, who carried out the Dec. 2 attack with his wife, Tashfeen Malik, 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.

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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 raided his home and questioned him for several days.

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 childhood, 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 before they were killed in a shootout with police a few hours later.

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Marquez, who had worked at Walmart and at a bar recently, also is related to Farook's family by marriage. His wife and the wife of Farook's older brother are sisters.

The San Bernardino 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, which has taken over large parts of Syria and Iraq, killed 130 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.

Farook and Malik were buried on Tuesday following a quiet, graveside Islamic ceremony guarded by FBI agents. Many fellow members of the mosque where they had worshipped refused to attend the service, two mosque members said.

President Barack Obama is due to travel to San Bernardino on Friday to meet privately with victims' families.