When Niels Högel, a nurse, worked shifts at two clinics in the northern German towns of Oldenburg and Delmenhorst in the early 2000s, the hospital rota showed a mysterious spike in patients who experienced sudden cardiac arrests and had to be reanimated.

Yet the bullish man in his late 20s was allowed to go about his business undisturbed until a colleague caught him in the act in 2005: for at least half a decade, Högel had systematically injected patients with cardiovascular medication in order to orchestrate a medical emergency and show off his resuscitation skills.

On Thursday, a district court in Oldenburg sentenced the now 42-year-old to a second life sentence in prison, revealing the full horror of his actions: Högel was found guilty of the murder of 85 patients in his care, some of them as young as 34, others as old as 96.

The ruling not only confirms Högel as the worst serial killer in the history of postwar Germany, but also raises uncomfortable questions about how a man colleagues referred to as “Lifesaver Rambo” managed to go unnoticed so long in an environment where the protection of vulnerable individuals should have been paramount.

Addressing Högel at the district court in Oldenburg on Thursday, the judge Sebastian Bührmann said that his crimes were “beyond comprehension”. “The human mind capitulates in the face of the sheer number of deeds,” he said.

Police suspect that Högel’s final death toll may be more than 200, while one spokesperson for a group of the victims’ relatives put the number as high as 300. But the court was unable to say for sure because of gaps in his memory and because many possible victims were cremated before autopsies could be performed.

He was handed the most severe form of life sentence possible under German law, which precludes the possibility of an early release after serving 15 years if the court judges there is a “severe gravity of guilt”.

After his arrest in 2005, Högel initially only went on trial for the murder of the 63-year-old patient he was caught injecting with a lethal dose of Ajmaline. But media attention encouraged further relatives to come forward, and Högel was jailed for his first life sentence in February 2015, for two murders and several attempted murders of intensive care patients at Delmenhorst hospital in Lower Saxony.

German police subsequently unearthed evidence of further murders after analysing scores of patient files and exhuming more than 130 bodies in Germany, Poland and Turkey, starting during his employment at another hospital and continuing after he was caught in the act by a colleague.

During a trial that started in October 2018, Högel was accused of 100 counts of murder. He was cleared of only 14 counts, and confessed to 43 murders. In 52 instances, he said he simply could not remember.

Some relatives of possible victims on Thursday said they struggled with the uncertainty of not knowing if their loved one was murdered or not. “It’s very, very tough,” said Frank Brinkers, who lost his father.

“I went through hell,” Brinkers told the German news agency dpa, adding that he had hoped his father’s case would turn out to be unequivocal. “It appears that wasn’t meant to be.”

In the last hearing before Thursday’s sentencing, Högel had asked the relatives of his victims for forgiveness. “I want to apologise wholeheartedly to every single one of you for what I have done over the years,” he said on Wednesday. Over the course of the trial, he said, he had come to realise the amount of suffering his “terrible deeds” had caused.

But many observers have started to question the sincerity of Högel’s statements. Until eventually confessing to some of the killings, the nurse had sworn on his daughter’s life that he had not killed anyone. One expert witness described Högel as a “competent liar”.

Gaby Lübben, one of the lawyers representing victims’ relatives, told the newspaper, Bild, that Högel’s apology was not credible: “He only acted out his remorse to gather plus points […] He should have stayed silent.”

A psychiatrist expert witness said during the trial that while the accused nurse had displayed traits of noticeable personality disorders, such as a lack of shame, guilt and empathy, they were not so severe as to nullify responsibility.

The question of whether Högel’s killings could or should have been prevented will be the subject of further court hearings. The state prosecutor believes that the nurse’s colleagues in Delmenhorst had enough clues pointing to the gruesome motive behind his actions from at least 9 or 10 May 2005 that they would have been “obligated to intervene”.

They only did so more than a month later, on 22 June, after Högel was caught in the act of administering a lethal injection to a patient.

Four of the nurse’s former colleagues at the Delmenhorst clinic, including two senior doctors, the head of the intensive care unit and one of his deputies, are yet to appear in court accused of manslaughter.

Christian Marbach, whose grandfather was found to be one of Högel’s victims in a previous trial, called the outcome a “milestone”, but insisted that further investigations into conduct at the two clinics were required. “Now it is very important that those who were in positions of power be brought to justice,” he said.

• This article was amended on 7 June 2019 to remove a phrase that was inconsistent with the Guardian style guide.