29/08/2017 9:28 AM AEST | Updated 29/08/2017 9:28 AM AEST

German Nurse May Have Killed Up To 180 Patients With Drug Overdoses, Police Said

Hundreds of bodies were exhumed to check for drug overdoses.

Fabian Bimmer / Reuters
Niels Hoegel, 40, has admitted that he sometimes sent patients into cardiac arrest out of

A German nurse convicted of murdering two hospital patients with lethal drug overdoses may have had a hand in the deaths of up to 180 others, in what police are calling Germany's worst killing spree since WWII.

The investigation by German police involved exhuming 130 bodies to test for lethal traces of heart medication, which the former nurse has confessed to injecting in patients to cause heart failure -- so that he could then attempt to revive them, and be hailed a hero.

Niels Hoegel, 40, was imprisoned for life in February 2015 for the murder of two intensive care patients and the attempted murder or causing bodily harm of four others at the Delmenhorst hospital in northern Germany, the ABC reports.

He worked as a nurse at a hospital in Oldenburg from 1999 to 2002, before moving to Delmenhorst Hospital in 2003, where he worked until investigations into his conduct began in 2005.

Former nurse Niels Hoegel hides he face from the media during the 2014 trial for two murders.

The number of suspected victims of Hoegel's crimes has been steadily climbing since investigations began in 2005, when a co-worker caught him administering an unauthorised injection to a patient at a Delmenhorst clinic.

In June 2016, investigators established the alleged involvement of Hoegel in 43 deaths. Now, they say they have established evidence for at least 84 killings, in addition to the two for which he's already been convicted.

Johann Kuehme, police chief in the north-western city of Oldenburg, said he was left "speechless" by the scale of the tragedy.

"And as if all that were not enough, we must realise that the real dimension of the killings by Niels H. is likely many times worse."

Kuehme admitted that police will likely never know the true scale of Hoegel's crimes, as many of the bodies of his potential victims will have been cremated, destroying the evidence.

The death toll "is unique in the history of the German republic," said chief police investigator in the case, Arne Schmidt, as he handed down the investigation's findings on Monday.

Hoegel had earlier testified that at times he had acted out of "boredom", News Corp reported. He said he felt euphoric when he succeeded in bringing a patient back to life, and devastated when he failed.

The scale of Hoegel's crimes will likely never be known, German authorities said as they presented the findings of their three-year investigation into the murders at a press conference in Germany on Monday.

Two former senior doctors and the head of the intensive care unit at Delmenhorst Hospital are now also facing charges of homicide by omission, according to HuffPost Spain.

Police chief Kuehme said many deaths could have been avoided if health authorities had acted on their suspicions sooner.

Prosecutors are expected to try Hoegel on at least some of the additional killings, according to ABC News, but Germany's judicial system does not allow for consecutive sentences, so future convictions will not affect his life term.