OLDENBURG (AFP) - German prosecutors Thursday (May 16) sought life in jail for a male nurse accused of murdering almost 100 hospital patients, making him the country's worst peacetime serial killer.

Niels Hoegel, 42, already behind bars for six killings, has confessed to giving scores more intensive-care patients drug overdoses because he enjoyed the thrill of trying to reanimate them at the last moment.

He stands accused of a revised toll of 97 murders, down from 100 as three cases could not be proved, prosecutor Daniela Schiereck-Bohlmann told the Oldenburg State Court.

Some investigators, however, believe Hoegel may have killed hundreds more by injecting them with deadly drugs while he worked at clinics in the cities of Oldenburg and nearby Delmenhorst between 2000 and 2005.

But because the deceased were buried or cremated long ago, autopsies have not been possible in all cases, and in some the post mortem examinations were inconclusive.

'Resuscitation Rambo'

During his trial, which has been running since October, the heavy-set and bearded defendant has admitted to 43 killings, denied five and not ruled out more than 50 others, saying he could not remember.

Prosecutors require clarity on each death because "just calling him the worst serial killer in history isn't enough to convict him," said Schiereck-Bohlmann, according to national news agency DPA.

Prosecutors say Hoegel was motivated by vanity, the desire to show off his skills at saving human lives, and simple boredom.

Some colleagues had reportedly nicknamed him "Resuscitation Rambo".

A psychologist testified in court that Hoegel suffers from a narcissistic personality disorder but could be considered fully culpable for his crimes.

At the start of the trial, Hoegel apologised to the families of the victims aged between 34 and 96.

"If I knew a way that would help you, then I would take it, believe me," he said last year. "I am honestly sorry."

130 bodies exhumed

Caught in 2005 while injecting an unprescribed medication into a patient in Delmenhorst, Hoegel was sentenced in 2008 to seven years in prison for attempted murder.

A second trial followed in 2014-15 under pressure from victims' families. He was found guilty of multiple cases of murder and attempted murder and given the maximum sentence, life imprisonment.

It was then that Hoegel confessed to his psychiatrist to dozens more murders at Delmenhorst, which prompted a far wider probe.

More than 130 bodies of patients who died on Hoegel's watch were exhumed in Germany, Poland and Turkey in a case investigators called "unprecedented in Germany".

Aside from the monstrosity of the killing spree, the Hoegel case has raised deeply troubling questions about how the hospital hierarchies failed to stop him for so long.

Statistics show that patient deaths, as well as the use of certain cardiac drugs, soared while Hoegel was on duty.

Several doctors and head nurses were later charged with manslaughter for failing to stop the killer nurse.

In the current trail, presiding judge Sebastian Buehrmann has ordered perjury investigations against some of Hoegel's former colleagues on suspicion they withheld evidence.

When the Oldenburg hospital encouraged Hoegel to resign in late 2002, it offered him a glowing letter of reference to ensure he left.

Hoegel later testified he was never explicitly told why the hospital wanted him gone but that the request made him feel as though he "had been caught".

"Without the mistakes of some people in Oldenburg... this series of murders by Niels Hoegel could have been stopped," Christian Marbach, whose grandfather was one of the victims in Delmenhorst, told AFP last year.

The verdict and sentencing are scheduled for June 6.