BERLIN, Jan. 22 (Xinhua) -- The public prosecutor of Oldenburg, Germany charged convicted serial killer Niels Hoegel for 97 additional counts of murder on Monday, Spiegel online reported.

The 41-year-old former nurse is already serving a life sentence for murdering six patients by lethal injection.

Hoegel has admitted to injecting his victims with drugs that caused, among other things, life-threatening heart failure or circulatory collapse, and then trying to revive them. The action was "out of boredom" and to impress his colleagues, according to German broadcaster Deutsche Welle.

Of the newly discovered cases, 62 patients died in the Delmenhorst hospital near the northern city of Bremen and 35 in a clinic in Oldenburg, Deutsche Welle said.

Hoegel committed the crimes largely between 2000 and 2005. He was first sentenced to 7.5 years in jail for attempted murder in 2008, and was handed a life sentence in 2015 over six counts of murder.

During his trial, Hoegel told a psychologist that he had killed scores more patients, which led to further investigation. A special commission of prosecutors and police exhumed more than 130 bodies and examined all deaths at his workplaces.

The death rate at Delmenhorst hospital's intensive care unit almost doubled while he worked in the department, according to Deutsche Welle.