During his trial, Hoegel testified that he intentionally caused cardiac arrest and other disruptions in about 90 patients in Delmenhorst, because he enjoyed the feeling of being able to revive them.
The convict himself claimed that he had killed over 80 patients in the intensive care units of hospitals in the cities of Delmenhorst and Oldenburg by giving them a lethal dose of medication.
The charges were expected after officials said in November that Hoegel may have killed more than 100 patients over several years.
As a result of his confessions, investigators carried out toxicological exams on dozens of patients who had died at the hospitals during the periods Hoegel's worked there - the results of these exams have led to the new charges being brought. "If the people responsible at the time, particularly at the Oldenburg clinic but also later in Delmenhorst, hadn't hesitated to alert authorities - for example, police prosecutors - Högel could have been apprehended sooner", Oldenburg Police Chief Johann Kühme said.
Niels H. was sentenced to life in prison in 2015 on two counts of murder and two counts of attempted murder and battery. Additional convictions could affect Hoegel's possibility of parole, but there are no consecutive sentences in Germany. In general, people serving life sentences are considered for parole after 15 years.
The Washington Post reports that while the exact number of murders can never be known, authorities have exhumed 134 bodies from 67 cemeteries in three countries, looking for traces of the deadly drugs.
Hoegel used a variety of drugs in his resuscitation attempts, Koziolek said.
Police believe Hoegel killed his first victim in early 2000 and then moved from hospital to hospital, managing to hide his killing spree with seeming ease.
Criminal charges are also being brought against other former members of staff who worked at the clinics during the time.