A German nurse serving a life sentence for murdering two patients is suspected of killing another 97 people by lethal injection.

If convicted, Niels Högel would become Germany's most deadly serial killer ever.

Högel, now 41, was charged with 97 further counts of murder on Monday. His third trial in the northern city of Oldenburg, Germany, is expected to start later this year, according to Reuters.

Police investigator Arne Schmidt told Deutsche Welle the killings are "unique in the history of the German republic."

Högel was sentenced to life in prison in 2015 on two counts of murder and two counts of attempted murder. But during his trial, he confessed to killing many others.

He admitted injecting 90 of his patients with a lethal cocktail of drugs in order to induce heart failure or circulatory collapse so he could try to revive them. Högel believed bringing dying patients back from the brink of death would help him shine as a hero and garner respect from his peers and superiors.

The New York Times reported Högel said he acted "out of boredom" and Reuters added that he described feeling euphoric after bringing someone back to life, although he often failed.

In statements to a psychologist he revealed over 30 killings — on top of the two for which he was convicted. That prompted authorities to launch an investigation and exhume scores of bodies buried between 1999 and 2005 and test them for the killer's drug cocktails.

As NPR's Laurel Wamsley reported in August, Högel's career as a nurse was ghastly throughout:

Also troubling is that Högel's lethal practices went unreported by fellow hospital staff. Though others noticed that the number of deaths in the intensive care unit at Delmenhorst doubled during his time there, nothing was done.

In 2005, a colleague saw Högel injecting a patient with ajmaline, but management didn't do anything about it for two days, during which Högel killed his final patient. Six employees at the hospital at Delmenhorst are now charged with negligent manslaughter for their failure to act. An investigation into neglect at Oldenburg is ongoing.

"The murders could have been prevented," [chief of police Johan] Kühme told the Guardian, noting that Högel was given a clean reference which allowed him to move to the Delmenhorst hospital and continue killing people. "People at the clinic in Oldenburg knew of the abnormalities."

On Monday, Oldenburg prosecutors told Reuters they had charged Högel with 97 additional murders but toxicology tests did not find conclusive proof in three more cases.

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.

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