INDIANAPOLIS (AP) — State prison officials say Orville Lynn Majors, who was convicted of killing six people while a nurse at an Indiana hospital, has died.

Indiana Department of Correction spokesman Douglas Garrison says the 56-year-old Majors died Sunday. A prison system statement says Majors was having breathing problems and became unresponsive at the Indiana State Prison in Michigan City. He was pronounced dead at a nearby hospital.

Prison officials say preliminary autopsy results show Majors had heart problems and died of natural causes.

Majors was serving a 360-year sentence. He was convicted in 1999 on charges he gave lethal injections of heart-stopping drugs to ailing patients at the former Vermillion County Hospital in Clinton, about 60 miles west of Indianapolis near the Indiana-Illinois state line.