Niels Hoegel, a former nurse who has admitted to the killing of 100 patients, is seen in court in Oldenburg, Germany, in March.
CNN  — 

German prosecutors in the northwest city of Oldenburg are seeking a life sentence for Niels Hoegel, a former male nurse who is considered Germany’s deadliest postwar serial killer after allegedly killing as many as 100 patients.

Lead prosecutor Martin Koziolek told CNN that 42-year-old Hoegel is suspected of taking that many lives, but there is evidence for only 97 murders.

In the three remaining cases, Koziolek said there may have been medical manipulations – but they are not believed to have directly led to the death of the patients.

Last year, Hoegel was charged with the deaths of 97 people. On his first day of trial in October 2018, Hoegel confessed that he killed his patients – ranging in age between 34 and 96 – at two hospitals in northern Germany between 2000 and 2005.

The former nurse is already serving a life sentence for six convictions, including homicide and attempted homicide in 2008 and 2015. Those convictions led authorities to investigate hundreds of deaths and exhume the bodies of former patients in the clinics where he worked.

Hoegel is accused of giving his victims various non-prescribed drugs, in an attempt to show off his resuscitation skills to colleagues and fight off boredom.

Prosecutors said Hoegel should have been aware that the drugs he gave to patients at hospitals in Delmenhorst and Oldenburg, in northwest Germany, could cause life-threatening cardiac problems.

In past hearings, Hoegel said he felt euphoric when he managed to bring a patient back to life, and devastated when he failed.

About 126 relatives of the victims are co-plaintiffs in the trial, which is expected to run until June this year, the prosecutor told CNN.