Hoegel, 41, was sentenced to life behind bars in 2015 for murdering two patients by giving them an overdose.
In 2005 Hoegel was caught injecting an unprescribed drug into a patient in Delmenhorst.
After confessing to murdering over 100 German patients, Högel is now one of the most prolific serial killers in Germany's modern history, and investigators are continuing to look for answers.
A commemorative plaque for victims of the nurse Niels Hoegel. He said his grandmother and his father, who were both nurses, had been his role models for going into the profession.
Hoegel then confessed to his psychiatrist at least 30 more murders committed in Delmenhorst, prompting investigators to revisit suspicious deaths in Oldenburg.
Oldenburg police chief Johann Kuehme a year ago said other medical workers at Oldenburg were aware of an elevated number of resuscitations, and initial indications of possible wrongdoing by the nurse in Delmenhorst emerged as early as April 2003.
The bearded, heavyset Hoegel took his seat in the courtroom wearing a hoodie and covered his face with a folder as long as photographers and camera teams were allowed to remain. "I hope he will be found guilty on each count so that the loved ones can finally find some closure", said Petra Klein, who runs a victims' support group.
The hospital in Oldenburg encouraged Hoegel to resign in late 2002, even offering him a glowing professional recommendation to ensure his departure.
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It added that the accident could be the result of "violations in the operation" of the ship fix facilities. He stated that the crane "has left a hole four or five meters" on the deck of the Admiral Kuznetsov .
Hoegel said he would get a euphoric feeling by saving patients, but felt devastated when he failed.
"Without the mistakes of some people in Oldenburg. this series of murders by Niels Hoegel could have been stopped", said Christian Marbach, whose grandfather was one of the victims in Delmenhorst.
Despite suspicion about the mounting deaths on Hoegel's watch, the hospital did not open an investigation.
Colleagues and superiors at the two clinics will be asked to testify in the current trial.
Records at the Oldenburg hospital showed rates of deaths and resuscitations had more than doubled when Hoegel was on shift, the German media reported.
During the same period, the use of medication for cardiac ailments soared.
Hoegel is believed to be responsible for the death of more than 130 patients, but the cremation of more possible victims makes determining the exact number of victims almost impossible, according to the British broadcaster BBC.
Relatives of his alleged victims packed the court.