A former nurse who already admitted murdering 13 patients in New Jersey hospitals pleaded guilty Wednesday to killing three other patients.

Charles Cullen (search) said little during his court appearance, during which he admitted slaying the patients in 1993 using a lethal dose of a heart medication.

Warren County Prosecutor Thomas Ferguson said that Cullen admitted during meetings on Friday that he killed Lucy Mugavero, 90, and Mary Natoli, 85, at Warren Hospital in Phillipersey and Pennsylvania counties.

Under the agreement, Cullen will not face the death penalty, but must help authorities solve all the crimes he committed in the 19 years between when he became a nursing student and last year, when he was charged with murder.

When his plea deal was announced last month, Cullen pleaded guilty to 13 murders and two attempted murders in Somerset County, N.J., and agreed to plead guilty to another murder in Northampton County, Pa., as well as to other cases officials could connect to him.

Law enforcement officials have met with Cullen several times since his April 29 guilty pleas to go over details of deaths they think he may have caused.

Cullen's pleas on Wednesday brought to 16 the number of patients he has admitted in court to killing. The admissions make him the deadliest serial killer in state history.

Howard Unruh, who killed 13 people in a 1949 shooting rampage in Camden, had been New Jersey's deadliest killer.

Under his plea agreement, Cullen, 44, will be sentenced to life in prison in both New Jersey and Pennsylvania, and will serve the sentence in New Jersey. He will not be eligible for parole for at least 127 years.

Cullen also agreed to meet with regulators trying to prevent future security breaches in hospitals.