Police say they have found evidence that Niels Högel, who was jailed for killing two patients, murdered more with lethal drug

At least 90 people were killed at the hands of a German nurse who injected patients with cardiovascular medication in order to show off his resuscitation skills, a three-year police investigation has found.

Niels Högel, 40, was jailed for life in February 2015 for two murders and several attempted murders of intensive care patients at Delmenhorst hospital in northern Germany.

But police have found evidence of another 88 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.

Since several of Högel’s patients were cremated, police said the real figure could be higher.

“The death toll is unique in the history of the German republic,” said the chief police investigator, Arne Schmidt, adding that Högel had killed randomly and preyed on those in a critical condition.

There was “evidence for at least 90 murders, and at least as many [suspected] cases again that can no longer be proven”, he told a press conference, declaring himself “speechless” at the outcome.

Police believe that the man whom the Bild newspaper is calling “Germany’s worst serial killer” carried out his first murder in February 2000, when he was still employed at a clinic in Oldenburg in Lower Saxony, close to the Dutch border.

After killing at least another 35 patients, he moved in 2002 to a hospital in Delmenhorst near the north-western city of Bremen, where he resumed his grisly practice within a week of starting his new job.

Högel would inject patients’ veins with a cardiovascular drug in order to orchestrate medical emergencies that would require him to step in and resuscitate them in the hospital’s intensive care unit.

The nurse used five different drugs including ajmaline, sotalol, lidocaine, amiodarone and calcium chloride, police said on Monday. Overdoses can lead to life-threatening cardiac arrhythmia and a drop in blood pressure, causing a rapid decline in an already ill patient.

During Högel’s time in Delmenhorst, the number of deaths at the hospital’s intensive unit doubled from about 5% to 10%, though the issue was not raised with authorities.

On 22 June 2005, a colleague at Delmenhorst hospital witnessed Högel injecting ajmaline into a patient, who died a day later. However, management decided not to call the police or raise the issue with their employee directly until two days later, allowing the nurse to kill another patient, his last, at 7pm on 24 June.

Six employees of the Delmenhorst clinic have been charged with manslaughter through failure to render assistance, while an investigation into neglect at the Oldenburg hospital is continuing.

“The murders could have been prevented,” said Oldenburg’s head of police, Johann Kühme. He added that those in charge could have acted faster to stop further loss of life. Instead, the nurse was given a spotless report that allowed him to continue his killing spree at another institution. “People at the clinic in Oldenburg knew of the abnormalities.”

When Högel was sentenced in 2008 to seven and a half years in prison for attempted murder, a woman who had followed the case in the media contacted police with suspicions that her mother could have also fallen victim to him.

The case was brought back to court, and in January 2015 Högel confessed to administering 90 unauthorised injections, of which 30 had been fatal because he had been unable to resuscitate the patients. At the time, he said he felt “fully responsible” for the 30 deaths but denied any further killings.

Konstantin Karyofilis, a psychiatrist, said last year that Högel was aware he had caused many people, including his patients and their families, “huge damage, suffering and anxiety”. He said the former nurse wanted it to be known that he was not “basking in the limelight” of his case. “This is not so. He is deeply ashamed,” he told the court.

As the extent of the nurse’s crimes has emerged, there have been calls for tighter controls on the use of drugs at clinics. The Högel case is the most extreme and bleakest of a number of similar instances that have shaken Germany’s healthcare system in recent decades.

In 2006, a male nurse was sentenced to life for the murder of 29 patients at a hospital in Sonthofen, Bavaria. In 2007, a nurse was sentenced for the murder of five of her patients at Charité hospital in Berlin.

Otto Dapunt, a former head of heart surgery at the Oldenburg clinic who worked with Högel for almost three years, told the court last year that the nurse had participated with an “above average regularity” in cases where resuscitation was necessary.

He said that while he had never considered this to be suspicious – particularly as the nurse was regularly on call and often had to deal with serious cases – he had often found Högel to be “overly zealous” in wanting to take care of the more critical patients.

He was also often unusually moved by the deaths of his patients, Dapunt said, recalling one occasion when the accused took two patients who had died to the morgue and returned in a “completely distraught state”.

“But he was factually competent, perhaps more competent than others,” Dapunt told the court.

Though Högel has already been sentenced to life in prison, the latest police findings mean it is likely he will face court again, with charges expected to be filed by spring next year.