Killer nurse Charles Cullen (search) on Monday admitted murdering five patients while he was working at Hunterdon Medical Center (search), bringing to 29 the victims whom the state's worst serial killer has confessed to slaying in New Jersey and Pennsylvania.

Cullen pleaded guilty to the murders in state Superior Court.

He also has acknowledged trying to kill five patients during more than a decade in hospitals and nursing facilities in the two states.

He has told investigators he might have killed as many as 40 people, which would make him one of the nation's most prolific mass murderers.

Cullen had previously denied killing anyone at Hunterdon Medical Center. As part of a plea bargain with prosecutors that enabled him to avoid the death penalty, Cullen has been cooperating with investigators in reviewing scores of suspicious deaths that occurred during his tenure at various hospitals to determine whether he had killed any other patients.

Cullen worked at Hunterdon's critical care unit from April 1994 until October 1996.

In most of the cases in which he admitted causing patient deaths, Cullen gave them an overdose of heart medication, usually digoxin.

He has admitted killing patients at Somerset Medical Center in Somerville; Warren Hospital in Phillipsburg; St. Barnabas Medical Center in Livingston and Morristown Memorial Hospital, as well as facilities in Easton, Salisbury Township and Fountain Hills, Pa.

He was arrested in December 2003.

He was able to move from hospital to hospital, despite suspicions he was killing patients, because the institutions did not report their fears to authorities.

In one instance, Somerset Medical Center (search) waited four months to notify authorities about its suspicions about Cullen, enabling him to kill five more patients there.

The hospital spoke with officials from the state's poison control center who bluntly warned them they had a serial killer on their hands, and urged the medical center to contact police.

Somerset said it believed the proper next step was to alert the state health department about possible laboratory irregularities regarding patient deaths there.