He was then sentenced to life in prison. Oldenburg police chief Johann Kuehme said Monday authorities have now unearthed evidence of 84 killings.
Kuehme said numerous deaths could have been prevented if the medical officials at the clinics in Oldenburg and Delmenhorst would have gotten law enforcement involved sooner.
Police told CNN they believe Hoegel killed 36 patients in Oldenburg between 1999 and 2001, and another 48 people from the hospital in Delmenhorst.
Hoegel's activity came to light in 2005 when a colleague witnessed him injecting a patient.
Police examined hundreds of deaths at hospitals where he had worked in the north-western city of Oldenburg and the nearby town of Delmenhorst.
When the news hit the headlines, another woman approached the police with suspicions that her deceased mother was also murdered by Hoegel. Kuehme said the number could be even higher, but because some former patients had been cremated, it would be impossible to tell.
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He said: 'Eighty-four killings leave us speechless.
According to investigators, however, the real number of victims is likely to be many times higher, but it cannot be proven because some of the deceased were cremated. Officials have stated that many deaths could have been avoided if authorities had been alerted earlier by staff at the Clinic.
He faulted local health authorities for being slow to act. He worked at a clinic in Delmenhorst, an urban district in Lower Saxony, at the time, ITV reported.
Hoegel admitted during his trial that he caused cardiac crises in approximately 90 patients so could resuscitate them. However, no evidence was found, and an investigation was subsequently launched.
Prosecutors are expected to try Hoegel on some of the killings but because Germany's judicial system does not allow for consecutive sentences, future convictions will not affect his life term.
The victims' names have not been released.