A prison assault on Aurora, Colorado, movie theater shooter James Holmes is among the reasons why authorities secretly transferred him in January to an undisclosed prison out of state, Colorado’s prisons director told ABC News.

At the time of the October prison assault, Holmes was housed at the Colorado State Penitentiary, the highest security prison in the state. The 28-year-old mass murderer was also being held in a pod by himself, with authorities closely monitoring him, according to ABC News. Despite these precautions, another inmate was able to slip through an open security gate and attack Holmes.

“We had not allowed Holmes at that time to come into daily contact with other offenders. That was not part of the management plan of him,” Colorado Prisons Director Steve Hager told ABC News. “The attack was part of the reason for moving him,” he added. “There were many concerns; the attack was part of the concern.”

According to documents obtained by ABC News, Holmes exited his case manager’s office on October 8 as a prison staff member slid open a nearby gate, leaving him within striking distance of inmate Mark “Slim” Daniels, a convicted car thief.

Colorado Department of Corrections

“[Daniels] ran through the slider, squeezing through as it was closing toward offender Holmes…[and] began hitting offender Holmes, in and around his head, with his fist,” a report said. According to the officer escorting Holmes, Daniels “landed at least two blows to offender Holmes before I was able to get behind [him].” Officers then struggled to subdue Daniels, with one prison staffer getting hit in her face and head in the melee.

ABC News reports that the officer escorting Holmes did not radio the officer who was controlling the gate because “his battery had gone dead,” according to an assessment of the incident.

As the public learned of the assault, Daniels began receiving letters of support, some of which included money. But he also received punishment in prison: 60 days in segregation, a 45-day loss of privileges and up to a year in restrictive housing, ABC News reports.

“I’m so sorry I couldn’t wipe him out and sent [sic] him packing to Satan’s lake of fire,” Daniels wrote, apologizing to the Aurora victims’ families in a letter to Denver’s Westword newspaper in December. “It was just impossible to do by myself with so many cops. I did get him six or seven good ones…. He was very scared.”

Holmes was convicted last July of opening fire at a Century Aurora 16 theater during the opening-night screening of The Dark Knight Rises on July 20, 2012, during which 12 died and 70 were injured. The court gave him multiple life sentences. Despite being housed at the prison for more than five weeks before the October incident, prison officials had yet to decide how they would hold him. One option was “protective custody” away from the other inmates.

After the October assault, however, Holmes was secretly moved in January to an undisclosed location, Colorado Department of Corrections spokeswoman Laurie Kilpatrick told the Associated Press. When asked by ABC News why his new location was being kept a secret, even from victims’ families, prison officials cited Holmes’s “security” as well as the well-being of prison staffers and other inmates.

According to the documents obtained by ABC News, prison officials believe that inmates will continue to target Holmes because of the high-profile nature of his crimes.