Washington DC - A man on death row who developed dementia during his 37 years awaiting execution has died in custody, apparently of natural causes, prison authorities said.

Ralph Leroy Menzies (67) was sentenced to death for the 1986 kidnapping and murder of a mother of three.

He was scheduled to be executed by firing squad in September, but the Utah Supreme Court blocked the execution after his lawyers argued that his dementia had become too severe.

"The Utah Department of Corrections is notifying the public that inmate Ralph L. Menzies passed away from presumed natural causes at a local hospital" on Wednesday, the agency said in a statement.

A judge had scheduled a new hearing for mid-December to reassess his mental state.

Menzies's attorney Lindsey Layer said over the summer, "We remain hopeful that the courts or the clemency board will recognize the profound inhumanity of executing a man who is experiencing steep cognitive decline and significant memory loss."

"Taking the life of someone with a terminal illness who is no longer a threat to anyone and whose mind and identity have been overtaken by dementia serves neither justice nor human decency."

Menzies would have been the seventh American prisoner executed by firing squad since 1977. He chose this method when given the choice decades ago.