Convicted mass killer James Holmes, who is serving multiple life sentences for gunning down a dozen people at a movie theater in 2012, was assaulted by another inmate at a high-security Colorado prison, a corrections official said on Friday.

Holmes was not injured when he was attacked by 27-year-old Mark C. Daniels at the Colorado State Penitentiary in Canon City, said Adrienne Jacobson, spokeswoman for the Colorado Department of Corrections.

Daniels lunged at Holmes and “took some swings” at him as the two prisoners were being escorted by guards down a hallway inside the lockup on Oct. 8, she said.

A corrections officer who got in between the two inmates during the dust-up and was not injured, she said, adding that Daniels will be charged administratively with assaulting Holmes and the officer.

Holmes, 27, was convicted in August of multiple counts of first-degree murder and attempted murder stemming from a July 2012 shooting rampage at a Denver-area multiplex that killed 12 moviegoers and wounded 70 others.

Prosecutors sought the death penalty for the California native, who had pleaded not guilty by reason of insanity, but jurors could not unanimously agree that he should be executed.

A judge sentenced Holmes to a dozen life sentences with no possibility of parole, plus a maximum 3,318 years for his conviction on attempted murder and explosive charges.

News about the prison assault was first reported by the Denver Post after Daniels sent a letter to the newspaper admitting to the attack, Jacobson said. Prison officials have not seen the letter, Jacobson said.

According to the department of corrections web site, Daniels has multiple felony convictions on his record. The Post reported that he is serving time for assault, menacing, smuggling contraband into prison and assaulting a prison guard.

Holmes was moved to the penitentiary in September, although corrections officials have said it is unclear where he will ultimately serve out the reminder of his life sentences because his high-profile makes him a security risk.

During an interview with a court-appointed psychiatrist that was videotaped and shown to jurors at the murder trial, Holmes said he could hear other prisoners at the jail where he was held calling him “a baby killer and stuff.”

The youngest victim killed in the rampage was six-year-old Veronica Moser, who was shot four times by Holmes.