A friend of Syed Rizwan ​Farook, one of the shooters who carried out the attack in San Bernardino on December 2, was arrested on Thursday for conspiring to provide material support to a terrorist plot, according to federal law enforcement and court documents.

Enrique Marquez Jr., 24, was charged on Thursday with the unlawful purchase of assault rifles used in the shooting, and also of conspiring to commit terrorist-style crimes with Farook as early as 2011 and 2012.

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According to the FBI, Marquez told investigators that in late 2011, he and Farook were planning to carry out terrorist attacks at Riverside City College and the eastbound lanes of State Route 91 during afternoon rush hour. The two allegedly planned to maximize casualties by throwing pipe bombs into crowds of students in the school library and cafeteria, and use pipe bombs to shut down a section of highway devoid of exits, then shoot motorists stranded in their cars. An affidavit said the two men went as far as purchasing firearms, ammunition, and tactical gear, as well as going to target practice at local shooting ranges, though the attacks never materialized.

Marquez reportedly converted to Islam in 2007, when he and Farook were neighbors in Riverside, NBC reported. Farook later introduced Marquez to radical ideology, telling his friend in 2011 of an interest in joining an al Qaeda affiliate in Yemen.

FBI officials said Marquez was not necessarily involved in the December 2 attacks, but that some firearms and explosive materials used in the attacks and found in the home of Farook and his wife, Tashfeen Malik, were purchased by Marquez.

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"Many dedicated law enforcement personnel have worked around the clock and side by side with their Joint Terrorism Task Force partners to gather evidence and build a solid terrorism case against Mr. Marquez over the past two weeks,” said David Bowdich, assistant director of the FBI's Los Angeles field office in a statement. "As we will continue to investigate the facts surrounding the terrible shooting in San Bernardino, we will leave no stone unturned in an effort to deliver answers and justice to the murdered victims and the families they left behind."

The development is the latest in a widespread investigation into the December 2 attacks, and into the couple who carried it out. Earlier this week, the FBI said that contrary to recent news reports, Malik had not posted publicly on social media about her support of violent jihad.