A former German nurse serving a life sentence for murdering patients by injecting them with a deadly medication is suspected of killing 84 other people, according to police.

If convicted, the man - identified only as Niels H under reporting rules - would be one of the worst serial killers in the country's criminal history.

A court order to exhume scores of former patients treated by the suspect found traces of the medication in 84 cases, Johann Kuehme the police chief in the northern German city of Oldenburg told a press conference on Monday.

Kuehme said police believe that the numbers of deaths could be even higher. However, many of the patients who died have since been cremated.

In past hearings, the nurse, now aged 40, admitted deliberately injecting patients at two clinics in northern Germany with deadly drugs and then trying to revive them in order to play the hero, German media said.

'Murders could have been prevented'

An Oldenburg court sentenced H to life in prison in February 2015 on two counts of murder, two counts of attempted murder and causing bodily harm to patients.

During his trial in 2015, the former nurse also confessed to murdering more patients during his career.

His confession prompted police to establish a special commission to investigate the deaths at two clinics where he worked - in Oldenburg and the nearby town of Delmenhorst.

The commission has been analysing hundreds of medical records and has exhumed more than 100 bodies to test them for traces of drugs.

In June 2016, police and prosecutors concluded that the man had killed 33 patients at the clinic in Oldenburg and that he had done so out of a desire to win approval by taking heroic measures trying to resuscitate them.

He injected patients with a drug so that they suffered heart failure or a shut down of their circulatory systems.

Two former senior physicians and the head of station at the Delmenhorst clinic are also facing manslaughter charges in relation to the deaths of the patients.

Investigators said their probe into the events at Oldenburg clinic is still ongoing.

"The murders could have been prevented," said Kuehme, with investigators believing that those responsible at the time could have acted faster.

"The clinic in Oldenburg knew about the irregularities," said Kuehme.