Correctional Service Canada says it is investigating the death of a 49-year-old inmate in New Brunswick.
In a statement it says the inmate was unresponsive when he was found in his cell at Dorchester Penitentiary last Thursday.
It says staff called emergency services and did CPR, but could not resuscitate him.
The inmate was in the medium security unit and the correctional service says he had been serving an indeterminate sentence since 1982 for first-degree murder.
Correctional Service Canada says it is reviewing the case and has notified police and the coroner as part of its standard procedure.