A former New Jersey nurse on Thursday pleaded guilty to murdering 13 patients and trying to kill two others in a deal that will spare him the death penalty.

Charles Cullen, 44, is expected spend the rest of his life in prison.

The former nurse, a quiet and frail man, has told authorities he killed as many as 40 patients in New Jersey and Pennsylvania over the past 16 years. He was arrested in December after officials at a hospital here became suspicious about the unexplained death of a patient.

His case has prompted calls for reforms of background checks on health care personnel. He had worked at 10 hospitals, was fired from several, and officials at some of the hospitals had raised questions about his handling of medications, but those concerns were never passed on to prospective employers.

As Judge Paul W. Armstrong of Somerset County Superior Court read the names of victims in the courtroom Thursday, relatives wept and hugged one another. Cullen wore a bulletproof vest under his beige shirt and spoke in a quiet voice as he agreed to the terms of his plea. Though prosecutors have said Cullen told them he killed the patients -- many facing difficult illnesses -- to end their suffering, the ex-nurse offered no explanation for his actions in court and expressed no remorse.

Emily Stoecker, whose mother-in-law, Eleanor Stoecker, died at Cullen's hands after he administered a lethal dosage of the heart medication digoxin, said she has yet to hear why he killed so many. "I wanted a reason why and I didn't get one," she said.

But Stoecker said she supported the plea agreement because it would provide additional information for many families who are wondering if Cullen killed their loved ones.

Under the deal, Cullen will be sentenced to two consecutive life terms and a mandatory 127 years in prison. The judge has not set a sentencing date.

Cullen's case prompted a complex investigation that spread across two states and involved prosecutors from seven jurisdictions. All signed off on Thursday's plea agreement, which Cullen sought to avoid the death penalty, in exchange for explaining how he pulled off his deadly run and eluded detection for so long.

Armstrong told Cullen that prosecutors can void the agreement and seek the death penalty if he refuses to cooperate with investigators. The agreement does not apply to other jurisdictions that might find him responsible for additional deaths.

Somerset County prosecutors first brought charges against Cullen in December for the murder of one patient and the attempted murder of another. After authorities took him into custody, Cullen, who last resided in Bethlehem, Pa., claimed to have killed as many as 40 patients, and officials said that number is not beyond the realm of possibility.

Cullen's attorney, public defender Johnnie Mask, declined to comment after the court hearing.

New Jersey Attorney General Peter C. Harvey said Cullen had been "on a killing spree." Last year, Harvey said, "he was killing people every month or attempting." Harvey told grieving families that "this is not the end but the beginning" of the investigation.

Officials said that the investigation will help produce reforms and statutes to strengthen oversight of the health care industry. "We need to know what he did and how he did it to discover the flaws in our system," Harvey said. "How did he get access to the drugs? How did he manipulate the computer system?"

In February, New Jersey officials fined Somerset Medical Center, where Cullen last worked, $10,000 for failing to immediately report the suspicious deaths. State officials said that the hospital's culpability will be explored through their investigation and by the numerous civil suits that have been filed by victims' families.

On Tuesday, New Jersey Gov. James E. McGreevey (D) signed a bill into law that requires health facilities to report medical errors to state officials. The Patient Safety Act will shield medical workers from civil liability if they report problems.

Former nurse Charles Cullen, left, stands as his lawyer Johnnie Mask reads the details of Cullen's deal with prosecutors sparing him the death penalty at a hearing in Somerville, N.J. Cullen pleaded guilty to murdering 13 patients and trying to kill two others.