Los Angeles: US authorities arrested on Thursday a man who bought the assault rifles used in the San Bernardino massacre that left 14 people dead, charging him with plotting terror attacks with Syed Farook, who was behind the rampage.

Enrique Marquez, 24, was charged with conspiring with Farook—a longtime friend and former neighbor—to commit two terrorist attacks in 2011 and 2012, though neither plot was ever carried out.

Marquez was also charged with the unlawful purchase of the guns used in the 2 December  shooting rampage carried out by US-born Farook and his Pakistani wife Tashfeen Malik, as well as the purchase of some of the explosive material used in the attack.

The three-count criminal complaint also charged Marquez with defrauding immigration authorities by entering into a sham marriage with a Russian woman who was a member of Farook’s extended family, US Attorney for the Central District of California, Eileen Decker, said in a statement.

Marquez made his initial appearance in federal court in Riverside, California, where he did not enter a plea but was appointed a lawyer.

‘Mr. Marquez conspired with Mr. Farook to commit vicious attacks,’ said Decker.

‘Even though these plans were not carried out, Mr. Marquez’s criminal conduct deeply affected San Bernardino County, Southern California and the entire United States when the guns purchased by Marquez were used to kill 14 innocent people and wound many others.’

The arrest marks a major development in the San Bernardino case being investigated by authorities as a terrorist assault—the deadliest since the September 2001 attacks on the World Trade Center.

Decker said there was no evidence Marquez took part in the attack on a holiday party at a social services center, or had prior knowledge.

However, she added: ‘His prior purchase of the firearms and ongoing failure to warn authorities about Farook’s intent to commit mass murder had fatal consequences.’

Investigations indicate that Farook had become radicalized several years before the 2 December  killings and before meeting his wife on a dating website.

Authorities said that on the morning of the assault in San Bernardino, Malik searched social media for information related to the Islamic State group, which has hailed the attack but stopped short of claiming it.

Shortly after the massacre, authorities say, a Facebook page associated with Malik proclaimed allegiance to IS leader Abu Bakr al-Baghdadi.

Farook and Malik, who had a baby daughter, died in a gun battle with police following the shooting.

‘Shy and mild-mannered’

Details are emerging of Marquez and his relationship to the killers.

He checked himself into a mental health facility after the massacre and posted a cryptic message on Facebook that said, ‘I’m. Very sorry guys. It was a pleasure,’ the Los Angeles Times reported.

Marquez, who had been working as a security guard at Wal-Mart, was known for his shy and mild-manned demeanor.

He was described as both impressionable and the type who ‘couldn’t fight his way out of a wet paper bag,’ the owner of a bar where he used to work told the Times.

According to the criminal complaint, Marquez met Farook around 2004 when he moved to Riverside.

Farook subsequently introduced him to Islam and in 2007 Marquez converted to the religion.

Shortly thereafter, Farook introduced him to radical Islam, prosecutors say.

The pair in late 2011 began planning to attack the library or cafeteria at Riverside Community College, where they had been students, with pipe bombs and guns, intending to inflict maximum casualties.

They also planned to attack with pipe bombs a major freeway in the region during rush hour.

‘After deploying the explosive devices, Farook planned to move among the stopped vehicles, shooting his rifle into them, and killing people,’ said authorities.

The pair later scrapped their plans, partly because of the arrests of several other suspects on terrorism charges during that period.

They grew apart but remained in contact and in November 2014 Marquez entered into a sham marriage with the Russian woman, for which he was paid $200 a month, authorities say.

Marquez faces up to 15 years in prison on the charge of providing material support to terrorists and up to 10 years each on the firearms and visa fraud charges.

He will be arraigned on 6 January, when he will enter a plea.