The rogue inmate, who managed to enter Breivik's personal section of Skien prison from the prison yard in April, hammered at the door of Breivik's cell while shouting death threats.
“If there wasn't a door between us, I'd kill you,” the intruder said, according to a letter Breivik sent to Norway's Press Complaints Commission.
Karl Hillesland, acting head of the prison confirmed to Aftenposten that the intrusion had indeed taken place.
“There was an incident. The inmate came to the “red door”, which is a security door. There was absolutely no danger of escape or physical contact,” he said.
Breivik wrote the letter to complain about the description of him in Aftenposten newspaper as a “mass murderer”.
Breivik was sentenced in 2012 to 21 years in prison for his brutal twin terror attacks on 22 July 2011, which cost 77 people their lives.