An ex-nurse has told US prosecutors that he killed 30 to 40 hospital patients over a 16-year period.
Charles Cullen said he killed patients to end their pain
Charles Cullen, 43, has been charged with one count of murder and attempted murder, but investigators are looking into his other self-confessed crimes.
Mr Cullen appeared in court in New Jersey following his arrest on Friday for the murder of a Catholic priest.
He told investigators that he performed mercy killings to end the suffering of terminally-ill patients.
Mr Cullen, from Bethlehem in Pennsylvania, was arrested and charged with the murder of a Roman Catholic vicar, the Very Rev Florian Gall, who had died of a heart attack at a medical centre in June.
After Mr Gall's death, it emerged that he had been given a lethal dose of the drug digoxin, used in the treatment of heart ailments.
Mr Cullen was also charged with the attempted murder of a 40-year-old woman at the same hospital.
During a 10-minute court hearing on Monday, he told the judge: "I am going to plead guilty. I don't plan to fight this."
He said he did not want a lawyer to represent him.
Under questioning following his arrest, Mr Cullen said he had killed many others at several hospitals in New Jersey and Pennsylvania during his 16-year career as a nurse.
Investigators are examining medical records at 10 hospitals where he worked in an effort to verify his claims.
Prosecutor Wayne Forrest said: "The evidence that we have indicates that may very well be the case."
Mr Cullen is in custody with bail set at $1m.