Convicted Indiana serial killer Orville Lynn Majors has died, according to the Indiana Department of Correction.

The 56-year-old native of Linton, Ind., was serving a 360-year sentence on six murder convictions.

Indiana State Prison correctional officers were assisting with a medical pass for Majors Sunday afternoon when he became unresponsive, according to a news release. Staff performed CPR and applied a defibrillator. LaPorte County EMS also performed CPR.

He was later pronounced dead at Franciscan Health Hospital in Michigan City, Ind.

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An autopsy conducted Monday revealed cardiac issues and determined he died of natural causes, according to the release.

Majors, a former licensed practical nurse, was found guilty in 1999 of the murder of six patients at the former Vermillion County Hospital whom he injected with heart-stopping drugs. Although Majors was convicted in six deaths, investigators compiled a list of more than 150 suspicious deaths at the hospital.

One nurse's review noted that Majors was working when 130 of 147 patients died over a 22-month period in the hospital's intensive care unit. More than a dozen bodies were exhumed during the 2 1/2-year investigation.

Some called him "The Angel of Death." During his sentencing, Judge Ernest Yelton called the murders "a paragon of evil."

Majors' earliest projected release date was listed as July 1, 2177.

Call IndyStar reporter Holly Hays at (317) 444-6156. Follow her on Twitter: @hollyvhays.

