An inmate who served roughly 35 years in solitary confinement in a federal prison died in a Colorado hospital earlier this month.

Thomas Silverstein, who was believed to have spent more time in solitary confinement than any other American federal prisoner, died from complications of heart surgery on May 11 at the age of 67, according to The New York Times.

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Silverstein had been brought to a hospital in February from the ADX Supermax prison in Florence, Colo., meant for the most dangerous criminals including gang leaders, terrorists and prisoners with persistent disciplinary issues.

Silverstein was first put in solitary confinement after killing a prison guard in October 1983 at the maximum security prison in Marion, Ill. He reportedly stabbed the guard to death, according to the Denver Post. Silverstein had also been convicted of killing two other prison inmates.

The Bureau of Prisons (BOP) put Silverstein in indefinite solitary confinement after the guard’s death, because at the time there was no death sentence for murdering a prison officer, writer Pete Earley explained in a blog post after Silverstein’s death.

Silverstein was initially incarcerated in 1978 for an armed robbery conviction. He was sentenced to 15 years in prison before the deaths of the two inmates and the prison guard.

After he was put in solitary confinement in 1983, he spent time in several prisons under various conditions, including cells in Atlanta and Leavenworth, Kan., with the lights turned on 24 hours a day.

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He was eventually allowed some privileges, Earley wrote, including books and art supplies.

In 2005, he was brought to the Supermax prison in Colorado and was kept in a soundproof cell.

According to his blog post, Earley had known and communicated with Silverstein for 32 years. The Denver Post reported Earley was working with Silverstein on an autobiography for the inmate.

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Though the BOP identified Silverstein as a member of the Aryan Brotherhood, Earley wrote in a second blog post that Silverstein denied it — despite other inmates who said he was.