Colorado mass shooter James Holmes was moved to a secret out-of-state location in the fall because he was viciously assaulted by a fellow inmate whose only regret was that he didn't do more damage, prison officials have revealed.

The attacker managed to reach Holmes by running through a closing security gate, an official at the Colorado State Penitentiary told ABC News, which first reported the story of Holmes' secret transfer.

Mark 'Slim' Daniels, a prisoner doing time for car thefts and other crimes, landed several punches on the mass killer's head and also struck a female officer before being subdued.

'The attack was part of the reason for moving [Holmes]. There were many concerns; the attack was part of the concern,' said Colorado Prisons Director Steve Hager to ABC News.

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Mass shooter James Holmes was moved to an undisclosed location after getting beat up by a fellow inmate last fall

Mark 'Slim' Daniels admitted to the assault on Holmes in a letter to a local newspaper, writing 'I'm so sorry I couldn't wipe him out'

A view of the outside of the maximum security Colorado state penitentiary in Canon City

The October 8 attack happened two months after Holmes was sentenced to multiple life sentences for opening fire in a movie theater during a 2012 screening of the Batman movie The Dark Knight, taking a dozen lives and injuring over 70 people.

Daniels was disciplined with 60 days in segregation, 45 days loss of privileges and up to a year in restrictive housing for striking Holmes, but also received fan mail and money from strangers, ABC News reported.

In a letter to a local newspaper, Daniels described the attack as an attempt to avenge the victims in Aurora.

'I'm so sorry I couldn't wipe him out and sent [sic] him packing to Satan's lake of fire. It was just impossible to do by myself with so many cops. I did get him six or seven good ones and this stupid ass case manager [name deleted] ran out of her office and and put her head in front of Holmes' cause I was about to knock him out. That's why she got socked a few times,' Daniels wrote, according to Westword.

James Holmes during his sentencing last August. The killer received multiple life sentences for opening fire inside a movie theater in 2012

According to ABC News, prison officials refused to say where Holmes was transferred, citing security concerns.

The lapse in security that led to the beating and the secret transfer drew criticism from a former Colorado prison official.

'It’s called a "dump job," moving him on to another warden. He’s always going to be a target. The Department of Corrections could easily have placed him in a secure setting,' Bob Hood, a former warden of the federal super-maximum security prison in Florence, Colorado told ABC News.

'It’s ironic to me that they transfer him out of state for such a simple assault. The Department of Corrections dropped the ball. The inmate was assaulted. The system broke down,' Hood said.