A former male nurse admitted murdering 100 patients as he went on trial in Germany on Tuesday, making him postwar Germany’s most prolific serial killer.

Niels Högel is accused of deliberately administering 100 patients in his care with lethal doses of medication between 2000 and 2005.

Prosecutors allege the 41-year-old deliberately put the patients in life-threatening situations so he could show off his resuscitation skills. The judge on Tuesday promised to shed light on the case, which he referred to as "a house of dark rooms".

Although there are no formal pleas in the German legal system, Mr Högel chose to admit the killing on the first day of his trial. Asked by the judge whether the charges against him were true, he answered: “Yes. All I have admitted is true.”

Mr Högel is already serving a life sentence after being convicted of the murder and attempted murder of six patients in similar circumstances. This is his third trial, but the first time he has been charged with mass killing.

He is accused of deliberately administering 100 patients between the ages of 34 and 96 with overdoses to cause cardiac arrest, so he could resuscitate them.

The killings took place at two hospitals in Oldenburg and Delmenhorst, both in north-western Germany. All of his victims were in intensive care at the time.

Mr Högel was first convicted of attempted murder in 2006, after he was caught by hospital authorities injecting a patient with unauthorised medication and reported to police. In the course of the first trial, evidence began to emerge of a pattern of behaviour.

Mr Högel allegedly boasted to fellow prison inmates that he had killed more than 50 patients. At a second trial in 2015 he was convicted of murdering two patients and endangering the lives of three others.

During that course of investigations, Mr Högel admitted to a psychiatrist that he had administered potentially lethal doses to around 90 patients and claimed around 30 had died.

In the run-up to the new trial, relatives agreed for the bodies of 130 of Mr Högel’s former patients to be exhumed and tested.

“We will do our utmost to learn the truth,” Judge Sebastian Buehrmann told the court on Tuesday.  “This case is like a house of dark rooms. We want to bring light into the darkness.”

More than 120 relatives of former patients are taking part in the trial as co-plaintiffs, and yesterday’s hearing had to be delayed to find space for them all in the courtroom.

Four former colleagues of Mr Högel are facing charges of failing to report evidence that could have prevented the killings.

A former heart surgeon at Oldenburg is alleged to have had Mr Högel transferred rather than report his concerns.

A former chief nurse is accused of giving Mr Högel a glowing reference despite later testifying that she knew doctors were concerned he was involved in too many resuscitations.

Two senior surgeons are accused of covering up an incident in which Mr Högel was caught administering a potentially lethal overdose to a patient rather than reporting it to police.

Mr Högel faces possible further life sentences if convicted. 

The trial continues.