As the BBC reports, 40-year-old Niels H is currently serving a life sentence for the two murders at a hospital in the town of Delmenhorst, in northern Germany.
Niels H administered the patients with lethal drug doses of heart medication in the unit where he worked. He was convicted of attempted murder and two counts of murder in 2015.
Authorities believe they have evidence from exhumed remains that links Niels H to at least 84 more deaths.
Judges believe Niels H injected patients with lethal doses because he enjoyed the feeling of being able to resuscitate them and play the hero, Reuters reports.
"Eighty-four killings ... leave us speechless," Police chief Johann Kuehme told reporters.
"And as if all that were not enough, we must realize that the real dimension of the killings by Niels H. is likely many times worse."
If the allegations are confirmed, the death toll could make Niels H. one of post-war Germany’s most prolific killers.