Enrique Marquez Jr. has been charged for conspiring to carry out earlier attacks and lying about the purchase of guns used in the December 2 shooting. He was previously indicted on terrorism charges.
The 24-year-old was charged on Wednesday with conspiring with the San Bernardino gunman Syed Farook to carry out attacks in 2011 and 2012 even though the pair never went through with their plans.
Marquez was also accused of making false statements when he bought the guns used in this month's deadly shootings.
The new charges follow a similar indictment on December 17, where he was charged with three counts of conspiring to commit a terrorist act, making an illegal gun purchase and committing immigration fraud in an alleged sham marriage to a Russian bride.
In new court documents filed on Wednesday, prosecutors said that Marquez and Farook planned to attack the college they attended with pipe bombs and guns and to gun down rush-hour commuters on a California freeway. But the plots were never carried out.
"Mr. Marquez is charged for his role in a conspiracy several years ago to target innocent civilians in our own backyard with cold-blooded terror attacks, and with providing weapons to an individual whose endgame was murder," David Bowdich, assistant director of the FBI's Los Angeles office, said in a statement.
Prosecutors say Farook and Marquez met in 2004 when they became neighbors in Riverside, California and that Farook introduced the younger man to radical Islamist theology.
No involvement in Dec shootings
Marquez is not charged with plotting the San Bernardino attack, which saw Farook and his wife, Tashfeen Malik, open fire during a Christmas party full of his co-workers at a social services center on December 2.
Fourteen people were killed and 22 others were injured in the shootings. The couple were killed in a gun battle with police hours later.
Authorities have said that the couple were inspired by Islamist extremism, calling it the deadliest such attack on US soil since the 9/11 attacks.
Meanwhile the US State Department said on Wednesday it had conducted all required security checks on Malik, but found no "derogatory" information that would prevent them issuing her a US visa.
Pakistan-born Malik entered the country under a K-1 "fiancee" visa. She was engaged at the time to Farook, a US-born citizen.
Investigators have said that before arriving in the US, Malik had sent private messages via social media outlining her interest in Islamic militancy.
mm/bk (AP, dpa, Reuters)