Federal authorities are expected to bring charges against a former neighbor of San Bernardino gunman Syed Farook who bought two of the guns used in last week's terror attack, Fox News has learned.

Enrique Marquez, who legally purchased the AR-15 rifles that Farook and his wife Tashfeen Malik used to kill 14 people and injure 21 others at a social services building Dec. 2, will likely be indicted after having provided substantial information to investigators about Farook, Malik and their acquaintances.

Law enforcement officials believe Farook and Marquez discussed executing an attack somewhere in the Los Angeles area in 2012, but decided not to do so after four people were arrested in nearby Riverside County in an unrelated terror case. Investigators believe Marquez bought the rifles and gave them or sold them to Farook in preparation for that planned attack. It is not clear what the target of the attack would have been. 

It was not immediately clear what specific charges Marquez would face, but Fox News has learned that he is the only person expected to be indicted anytime soon in connection with the attack. However, authorities say that other subjects are being monitored.

Marquez, 24, began speaking with federal authorities after they raided his mother's home over the weekend. Right after the shootings, Marquez called his mother to say he was safe, but that he wouldn't be coming home, neighbor Lorena Aguirre told the Associated Press. He later checked into a mental health facility. It's unclear where he is now.

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Marquez is also related to Farook by marriage, having wed the sister-in-law of the gunman's older brother, state records show. The Associated Press reported that Marquez and and his bride, Mariya Chernykh listed their address on the marriage license at the same Corona home where Syed Raheel and Tatiana Farook live. Viviana Ramirez, a friend of Marquez, told the Los Angeles Times that Marquez rarely spoke about his family or his marriage.

Tommy Lopez, who said he became friends with Marquez after meeting at a punk rock show seven months ago, told the Associated Press Marquez never talked about his wife and was not religious.

However, Marquez had either converted or was in the process of converting to Islam, though officials at the Islamic Society of Corona-Norco, where his marriage ceremony supposedly took place, described his attendance as infrequent and denied that he had gotten married there. 

Azmi Hasan, the mosque's facilities manager, said Wednesday that he understood Marquez had converted to Islam, but said he was not a member of that mosque. Marquez had only worshipped there three to four times over seven years, said Hasan, who hadn't seen him in about four years.

Marquez had a security guard license in California for several years, but it expired last year. He had worked at Wal-Mart since May, but has since been fired, spokesman Brian Nick told the Associated Press.

He spoke of wanting to join the military, Ramirez, a fellow student at Riverside Community College told the Los Angeles Times. His brother-in-law, Raheel Farook, is a Navy veteran, serving from 2003 to 2007 and earning the Global War on Terrorism Expeditionary Medal and the Global War on Terrorism Service Medal, among other awards.

Investigators were also examining the digital footprint left by the shooters, who tried to destroy their computer hard drives and cell phones prior to carrying out the attack. Investigators tell Fox News the data that has been recovered shows that Syed Farook posted multiple messages online expressing support for Islamist groups like Muslim Brotherhood and Hamas, while also expressing hatred for Israel. Investigators have also expressed concern that Farook may have been in contact with other terror groups. 

The FBI investigation into the mass shooting has been one of the most comprehensive in the bureau's recent history. Law enforcement sources tell Fox News that more than 300 interviews related to the shooting have taken place in the U.S. An FBI team has also been dispatched to Europe as part of the investigation.

Fox News Channel's Adam Housley and the Associated Press contributed to this report.

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