(CNN) Thomas Silverstein, a triple murderer who spent decades in solitary confinement in federal prison, died at a Colorado hospital this month, officials said Wednesday.

Silverstein died May 11, according to the Federal Bureau of Prisons. He was 67.

He had lived in solitary confinement since killing a prison guard in 1983, a span of more than 35 years. Much of that time he was subject to an unusual no-contact order in which he had little interaction with other people.

Silverstein is believed to have served the longest term in solitary confinement in the federal penitentiary system.

He was first sent to prison in 1978 after a robbery conviction. While at a facility in Kansas, he was accused of killing an inmate and moved to a maximum-security prison in Marion, Illinois. His first murder conviction was overturned, but while in Marion he killed two inmates and fatally stabbed a guard.

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