SOMERVILLE, New Jersey (CNN) -- A former nurse accused of murdering dozens of patients in New Jersey and Pennsylvania hospitals over nearly two decades pleaded guilty Thursday to 13 counts of murder and two counts of attempted murder.
As part of the plea agreement, Charles Cullen will be sentenced to 13 life sentences and two 20-year sentences, which would allow him to avoid the death penalty. He will be sentenced at a later date.
Looking thin and pale and wearing a tan shirt, Cullen politely answered New Jersey State Superior Court Judge Paul W. Armstrong's questions and listened as the judge read aloud the names of the 15 victims.
"The court is satisfied that Mr. Cullen is making a known and intelligent and a voluntary plea," Armstrong said.
Several family members of patients who died under Cullen's care were in the courtroom.
Meanwhile, other New Jersey and Pennsylvania counties where the 44-year-old Cullen worked continued to review their hospital records, looking for possible victims.
John Haggerty, spokesman for the New Jersey attorney general's office, which helped broker the plea agreement, said earlier this week that the deal means Cullen "will provide cooperation in the investigation of murders he may have committed in hospitals and nursing homes in seven counties in New Jersey and Pennsylvania from 1984 to 2003."
Cullen claimed to have killed between 12 and 15 patients at Somerset Medical Center, where he worked for several years, and as many as 40 patients at 10 other medical facilities where he worked, Somerset County prosecutor Wayne Forrest told reporters in December.
Cullen was initially charged with murder in the death of a Catholic priest who entered Somerset Medical Center for treatment of heart disease. The priest was found to have lethal levels in his system of the drug digoxin, a heart medication.
The original attempted murder charge involved a female patient who had cancer and heart disease. She recovered from an overdose of the same drug that killed the priest, Forrest said.
Cullen was fired from the Somerset Medical Center after an investigation of six patients with abnormal lab results showed him as the only common denominator.