An inmate at the “Supermax” prison in southern Colorado, who had been held in solitary confinement longer than any other federal prisoner, has died at the age of 67.

Thomas Silverstein, who was held in solitary confinement for 35 years, was admitted to a suburban Denver hospital in February to undergo surgery and died in intensive care May 11.

He entered prison in 1978 on an armed robbery conviction and was later found guilty of killing two inmates and a prison guard. The Denver Post reports that with no federal death penalty in place at the time of the guard’s slaying, the Bureau of Prisons instead put Silverstein in indefinite solitary confinement, where he remained until he was hospitalized.

He was moved to “Supermax” in Florence in 2005 and was housed in a soundproof cell.