A German former nurse serving a life sentence for two murders is suspected of killing dozens more patients by injecting them with heart medication, investigators say.

A court order to exhume 99 former patients of the man - named only as Niels H - found traces of heart medication in 27 bodies, they say.

Niels H, 39, was convicted in February last year over two patients' deaths.

In court he admitted killing up to 30 patients with heart medication.

The judges at his trial at Oldenburg district court in northern Germany said that he had a desire to win approval by resuscitating patients. He gave them overdoses of a drug that shut down their cardiovascular systems.

The deaths took place between 2003 and 2005. Police in April said they were investigating at least 200 deaths, including at other clinics where he worked, in Oldenburg and Wilhelmshaven.

About 30 of the 99 exhumed bodies of former patients at the hospital in Delmenhorst were found to have heart drug residues, police told Der Spiegel (in German).

"We assume that the actual number of victims is much higher," the magazine quoted a police spokesman as saying.

Investigators say that their task is made harder because Niels H himself cannot remember how many patients he injected.

Prosecutors say there is now a strong probability that the defendant will face a new indictment.

If found guilty of all the deaths, he would become one of Germany's worst post-war serial killers.

During the trial one senior doctor described Niels H as a "passionate medic" who had made a good impression on staff.

Doctors did notice, however, that he always seemed to be around when patients were being resuscitated, often assisting junior doctors with the procedure, he added.

Although he was charged with three murders, the court found him guilty on only two counts, explaining that it could not be proved that the former nurse had been responsible for the third death.

In his trial last year he said he was "honestly sorry" and hoped families would find peace for his crimes, which he said were "relatively spontaneous".

A psychiatric expert in January 2015 said that Niels H - who was convicted of attempted murder in 2008 and imprisoned for seven and a half years - had claimed to have over-medicated another 90 patients, 30 of whom died.