Niels H nurse murderer Oldenburg Germany
Former nurse Niels H. (L) masks his face with a folder with the help of his lawyer Ulrike Baumann on his arrival in the courtroom at the regional court in Oldenburg (file photo) REUTERS/Fabian Bimmer

A nurse who was convicted of murdering two patients in northern Germany with overdoses of heart medication is believed to have killed at least another 84 people, according to police.

Niels H was charged in 2015 for the murders of two people along with two attempted murders at a clinic in the north-western town of Delmenhorst – though prosecutors have long presumed he killed more people and put the figure to 43 at least last year.

Investigators now say that the the true scale of the killings could be larger as the crimes came to light after Niels was convicted of attempted murder in another case. Authorities subsequently investigated hundreds of deaths and exhumed former patients in Delmenhorst and Oldenburg.

The 40-year-old's victims received lethal drug doses at units where he worked, and relatives of patients who died in clinics where he worked urged police to investigate further.

A commission was set up in 2014 to investigate the scale of his crimes, which could make him Germany's worst post-war killer, with judges claiming he was motivated by a desire to win approval by resuscitating the patients he had drugged – causing them heart failure or collapse of their circulatory systems.

Hundreds of medical records have been analysed, with 134 bodies being exhumed to test samples for drug residue. The investigations have been made more difficult because many patients were cremated.

Known as just Niels H due to German reporting rules, he admitted to applying the drug to approximately 90 people at an intensive care clinic in Delmendorst during his 2015 trial. The abnormalities go back to 2000 at another clinic in Oldenburg, police say.

Johann Kuhme, an Oldenburg police chief, said: "The findings continue to breach any imagination. It is simply not possible to say how many people were killed."

The new charges are likely to be filed at the beginning of 2018, and Kuhme warned that the number could rise furthermore.