A federal prisoner and triple murderer who is thought to have spent the most time in solitary confinement has died at age 67.

Thomas Silverstein, an inmate at the Supermax prison in Colorado, reportedly died at a hospital on May 11. He spent more than 35 years in solitary confinement and subject to an additional order that meant he had even less contact with others than other solitary-confinement inmates, CNN reports.

Silverstein was first sent to prison in 1978 after being convicted of robbery. He was accused of killing an inmate in Kansas, but after moving to a maximum security prison in Illinois, the conviction was overturned.

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He later killed two inmates and fatally stabbed a prison guard in Illinois.

Before moving to Colorado, Silverstein was held in Atlanta and Kansas. In Atlanta, he was kept in a 6-foot-by-7-foot cell under nonstop surveillance.

While in Supermax, he watched TV, drew, crocheted, did yoga and meditated, according to CNN.

Silverstein’s attorneys sued the prison bureau in 2007, calling his treatment "cruel and unusual" punishment.

"It's almost more humane to kill someone immediately than it is to intentionally bury a man alive," Silverstein reportedly wrote in a 2008 letter to his friend, according to CNN.