James Holmes, the Colorado movie theater shooter who was jailed for life last month for murdering 12 people, has been attacked behind bars.

Holmes, 27, who is currently being housed at the Colorado State Penitentiary, was attacked by fellow inmate Mark 'Slim' Daniels, also 27, earlier this month.

The Colorado Department of Corrections confirmed the attack took place while Holmes, who is currently being kept isolated from the other prisoners, was being moved between cells.

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Mark 'Slim' Daniels, 27, who is doing time for assault and auto robbery, is now charged with attacking Colorado shooter James Holmes, also 27, as the pair passed each other in a prison hallway

According to the Denver Post, Daniels, who is serving time for assault, auto theft and menacing, laughed at Holmes then took several swings at him as the pair passed each other in a hallway.

Daniels will now be charged with assault on Holmes and the officer who was escorting him, despite the fact that Holmes was apparently not injured.

Daniels has been charged with assaulting an officer once before back in 2006, and is also serving a sentence for bringing contraband into prison.

According to the Denver Post, the paper was sent a letter purporting to be from Daniels saying that the attack happened on October 8, but officials would not confirm this.

Homles is being kept isolated on a wing at maximum-security Colorado State Penitentiary (pictured) but was being moved from one part of the jail to another when he crossed paths with Daniels

Holmes is currently serving 12 life terms plus 3,000 years without the possibility of parole at Colorado State Penitentiary, the highest-security jail in the state.

He was convicted of killing 12 people and wounding another 70 after opening fire in a movie theater in Aurora during a screening of The Dark Knight back in 2012.

Holmes had been facing the death penalty when he was sentenced last month, but a lone juror held out, meaning he was given a prison term instead.

Many of the relatives of Holmes' victims criticized the decision at the time, saying he should have been punished to the fullest extent of the law.

According to one woman, who identified herself only as Juror 17, one woman was firmly in favor of a life sentence because of concerns over Holmes' mental health.