The Joint Forces Penitentiary Squad has charged David Bagshaw, 21, with attacking another inmate with a weapon last month.
The charge stems from a violent clash inside the gymnasium at the Bath, Ont., facility on March 20, which left one man dead and two others injured.
A 34-year-old victim suffered severe injuries in the melee.
Another man, Jordan Trudeau, 29, died after being shot in the chest during the same melee.
Bagshaw was also shot by officers during the melee but his wounds were not life-threatening.
A police investigation has determined that prison officers followed proper protocol during the violence at Millhaven.
Bagshaw was sentenced to Millhaven for the murder of 14-year-old Stefanie Rengel.
Bagshaw was four days shy of his 18th birthday when, on New Year's Day 2008, he lured the young girl from her home and stabbed her six times with a kitchen knife, leaving her to die in the a snowbank.
The Joint Forces Penitentiary Squad, made up of officers from the provincial police, Kingston police and RCMP, continues to investigate the incident.
With files from The Canadian Press
