This photo provided by the Ohio Department of Rehabilitation and Correction shows Donald Harvey, a serial killer who became known as the "Angel of Death" and pleaded guilty in 1987 to 37 murders of hospital patients while working as a nurse's aide in Cincinnati and London, Kentucky, during the 1970s and '80s, claiming he was trying to end his patients' suffering. A spokeswoman for Ohio's prison system says Harvey was found badly beaten Tuesday, March 28, 2017, in his cell at the state's prison in Toledo, and the Ohio State Highway Patrol said Harvey was in critical condition Wednesday. He is serving multiple life sentences as part of a plea deal that avoided the death penalty. (Ohio Department of Rehabilitation and Correction via AP)

This photo provided by the Ohio Department of Rehabilitation and Correction shows Donald Harvey, a serial killer who became known as the "Angel of Death" and pleaded guilty in 1987 to 37 murders of hospital patients while working as a nurse's aide in Cincinnati and London, Kentucky, during the 1970s and '80s, claiming he was trying to end his patients' suffering. A spokeswoman for Ohio's prison system says Harvey was found badly beaten Tuesday, March 28, 2017, in his cell at the state's prison in Toledo, and the Ohio State Highway Patrol said Harvey was in critical condition Wednesday. He is serving multiple life sentences as part of a plea deal that avoided the death penalty. (Ohio Department of Rehabilitation and Correction via AP)

TOLEDO, Ohio (AP) — A serial killer known as the “Angel of Death” after he admitted killing three dozen hospital patients in Ohio and Kentucky during the 1970s and ’80s was beaten in his cell and is in critical condition, state authorities said Wednesday.

Donald Harvey, who is serving multiple life sentences, was hospitalized, said JoEllen Smith, spokeswoman for Ohio’s prison system.

He was found in his cell Tuesday afternoon at the state’s prison in Toledo and was in critical condition Wednesday, the State Highway Patrol said. While details about his injuries weren’t released, he was beaten when an unnamed person went into his cell, a patrol report said.

Harvey, 64, pleaded guilty in 1987 to killing 37 people, mostly while he worked as a nurse’s aide at hospitals in Cincinnati and London, Kentucky. He later claimed he was responsible for killing 18 people while working at the Veterans Administration Medical Center in Cincinnati.

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He told his former attorney the killings began in 1970 when was at Marymount Hospital in Kentucky.

Many of his victims were chronically ill patients and he claimed he was trying to end their suffering.

Harvey used arsenic and cyanide to poison most of his victims, often putting it in the hospital food he served them, prosecutors said. Some of the patients were suffocated when he let their oxygen tanks run out.

He was caught after a medical examiner smelled cyanide while performing an autopsy on a victim.

Harvey told a newspaper after he pleaded guilty to avoid the death penalty that he liked the control of determining who lived and died.

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This story has been corrected to show that Harvey pleaded guilty to 37 murders, not 35.