Federal authorities are set to file criminal charges later today against the friend who investigators say supplied guns to the married couple behind the deadly San Bernardino, California, attack earlier this month, according to reports.

CNN and NBC said the charges are expected to be filed against Enrique Marquez, a friend and former neighbor of radicalised Muslim Syed Rizwan Farook, who carried out the December 2 attack with his wife, Tashfeen Malik, that killed 14 people and 21 injured.

Marquez, 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 legally purchased the two AR-15 assault-style rifles that Farook and Malik 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.

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.