Oldenburg police chief Johann Kuehme said: "Eighty-four killings... leave us speechless".
Police say the number of victims may be even higher but the final number may never be known as some possible victims were cremated.
Hoegel would inject patients with an overdose of unprescribed heart medication in order to revive them and show off his nursing skills, a court heard during his initial trial.
Kuehme added that if authorities acted sooner, some of these deaths could have been prevented.
Hoegel is already sentenced to jail for life.
'And as if all that were not enough, we must realise that the real dimension of the killings by Niels is likely many times worse'. He worked at a clinic in Delmenhorst, an urban district in Lower Saxony, at the time, ITV reported. As part of the new investigations, the police exhumed bodies of the patients that were declared dead at the hospital where Hoegel used to work.
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Police told CNN they believe Hoegel killed 36 patients in Oldenburg between 1999 and 2001, and another 48 people from the hospital in Delmenhorst.
The daughter of a deceased woman came forward suspecting Hoegel's involvement in her mother's death, leading to an investigation which saw him admit guilt in 30 cases, naming the patients he killed.
Following the confession, relatives of patients who died in the clinics where Hoegel worked urged German police to investigate the horrific claim. All to impress co-workers with his resuscitation skills. The investigations have been made more hard because many patients were cremated. He said he liked the feeling of bringing them back.
Prosecutors are expected to try Hoegel on at least some of the additional killings but Germany's judicial system does not allow for consecutive sentences, so future convictions won't affect his life term.
The identities of the suspected 84 victims have not been released and no immediate comments were issued on Monday from any representatives of victims' families.
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