The man, identified only as Niels H. under reporting rules, has "confessed in many of those cases", a police statement said, but could not remember all the details of his actions. If confirmed, the death toll would be among the worst ever compiled by a German serial killer.
In past hearings, Niels H. admitted deliberately injecting patients at two clinics in northern Germany with deadly drugs and then trying to revive them in order to play the hero, German broadcaster NDR said.
He was convicted of two charges of attempted murder and two counts of murder by an Oldenburg court in 2015.
Police said on Monday that they had investigated additional deaths at hospitals in the northern German cities of Oldenburg and Delmenhorst after exhuming the remains of 134 people with links to Niels H.
They said he had used five different drugs on the patients, including alkaloid ajmaline and arrhythmia drug sotalol, between 1999 and 2005.
Prosecutors have also charged six people who worked with Niels H. at the Delmenhorst hospital on suspicion of failing to stop the killing even though they were aware of it.
Ten years ago, a German nurse was convicted of killing 28 elderly patients. He said he gave them lethal injections because he felt sorry for them. He was sentenced to life in prison.
In Britain, Dr. Harold Shipman was believed to have killed as many as 250 people, most of them elderly and middle-aged women who were his patients. Known as Dr. Death, Shipman was sentenced to 15 life terms in 2000; he died prison in 2004, apparently a suicide.