BERLIN — A German nurse serving a life sentence for murdering two of his patients is believed to have killed at least 86 people entrusted to his care, officials said Monday, in what they described as an imagination-defying series of crimes.

The nurse, identified as Niels Högel, was sentenced to life in prison in February 2015, after he confessed and a court in the northern town of Oldenburg found him guilty of administering overdoses of heart medication to some patients in an intensive care ward in Delmenhorst. He was convicted of two counts of murder, two counts of attempted murder and causing bodily harm to patients and is serving his sentence.

During his trial, the former nurse had also confessed to intentionally inducing cardiac crises in 90 of his patients, 30 of whom he said had died. That prompted officials to launch an investigation into the deaths of some 130 of Mr. Högel’s former patients. The results were presented on Monday in Oldenburg.

At least 84 of the convicted killer’s former patients were found to have died after suffering from injections of five different forms of medication, Johan Kühme, the chief of police in Oldenburg told reporters.