CBS Local — A nurse in Germany who was convicted of fatally overdosing two patients has now been linked to the killings of at least 84 other people. Investigators believe the male nurse may be responsible for even more overdoses, making him one of the country’s worst serial killers.

Niels Hoegel is already serving a life sentence following his 2015 trial in Oldenburg, Germany. During the trial, Hoegel reportedly admitted that he had triggered cardiac arrests in about 90 of his patients. He allegedly added that dozens of those victims died from his actions.

Following the confession, relatives of patients who died in the clinics where Hoegel worked urged German police to investigate the horrific claim.

“The findings continue to breach any imagination,” Oldenburg police chief Johann Kuhme said. “It is simply not possible to say how many people were killed.”

Police have reportedly exhumed over 130 bodies who died in the clinics in Delmenhorst and Oldenburg. They are being tested for drug residue that may reveal if they were victims of Hoegel’s overdoses. Many of the 40-year-old’s suspected victims were cremated making the new investigation that much harder.

Prosecutors say they suspect Hoegel had killed many of his patients by injecting them with unneeded heart medication or toxic doses of potassium. Oldenburg police added the number of suspected victims could rise. New charges are likely to be filed against the former nurse in 2018.