MICHIGAN CITY, Ind. (WANE) Orville Majors, the Indiana hospital worker sentenced to 360 years in prison for the killings of at least six patients, died at the Indiana State Prison in Michigan City Sunday afternoon. He was 56.

Correction officers found Majors breathing heavy in his cell around 4:30 p.m. Sunday, and declared a medical emergency when he became unresponsive. The staff performed CPR on Majors and he was shocked with a defibrillator before he was transported to the Custody Hall where LaPorte County EMS continued CPR.

Majors was pronounced dead at Franciscan Health Hospital in Michigan City by the La Porte County Coroner just more than a half-hour later, at 5:11 p.m. on Sunday.

An autopsy showed Majors died of cardiac issues and his death was found to be of natural causes, the Indiana Department of Corrections said.

Nicknamed the “Angel of Death,” Majors was serving a 360-year sentence for the deaths of six elderly patients at Vermillion County Hospital in Clinton, Indiana in 1994 and 1995. Majors, a registered nurse at the small hospital, injected the patients with potassium chloride.

The charges came out of what was one of Indiana’s greatest criminal investigations, launched after the hospital charted a 400 percent increase in its death rate – primarily in the intensive care unit Majors worked in.

While Majors was only convicted of six killings, investigators believed he may have committed more than 100 between 1993 and 1995.

Majors’ earliest projected release date had been July 1, 2177.