Charles Cullen, 43, has been charged with one count of murder and attempted murder, but investigators are examining records at 10 hospitals where he worked in attempt to document his claims about the other killings.
"The evidence that we have indicates that may very well be the case," prosecutor Wayne Forrest said.
Cullen, 43, told investigators that he performed mercy killings to alleviate the pain of patients who were "very sick," Forrest said.
During a 10-minute court appearance Monday, Cullen stood and told the judge, "I am going to plead guilty. I don't plan to fight this." He said he did not want a lawyer.
Cullen, of Bethlehem, Pa., was arrested Friday and charged with murder in the death of a Roman Catholic clergyman who was a patient at Somerset Medical Center. He was also charged with the attempted murder of a 40-year-old woman at the same hospital.
Cullen told authorities he had administered overdoses doses of drugs to the patients and had killed many others in his 16-year nursing career.
The Very Rev. Florian J. Gall, vicar of Hunterdon County in the Catholic Diocese of Metuchen, went into cardiac arrest on June 28 at Somerset Medical Center. It was later determined that he had a lethal level of the drug digoxin, a heart medication, and had died from an unauthorized administration of the drug.
On June 16, the 40-year-old heart and cancer patient was discovered with an elevated level of digoxin. The woman recovered from the overdose and was discharged from the hospital, but died in September.
Cullen, a divorced Navy veteran, lived with a girlfriend who is a nurse in Pennsylvania, Forrest said.
Neighbors in Cullen's Bethlehem neighborhood described him as quiet and said he rarely interacted with them since moving to the area a few months ago.
Kristy Hendershot said her father recently spoke with Cullen, who gave no indication anything was amiss.
"When they were shoveling snow, (Cullen) talked to him like everything was fine," she said. "He seemed like a friendly guy. I didn't see him come out of his house a lot."
Cullen was ordered held on $1 million bail.