Somerville, N.J. ? A former nurse was charged with murder Monday after telling prosecutors that he has killed 30 to 40 severely ill patients in Pennsylvania and New Jersey since 1987 by injecting them with drugs.

Charles Cullen, 43, told investigators that he administered the drug overdoses to put “very sick” patients out of their misery, prosecutor Wayne Forrest said.

If Cullen’s claims prove true, it would be one of the biggest hospital murder cases in U.S. history.

Cullen was charged with one count each of murder and attempted murder, but more charges could follow. Investigators are examining records at 10 hospitals where Cullen worked as they try to document his claims about the other killings.

“The evidence that we have indicates that may very well be the case,” Forrest said.

In court Monday, Cullen stood and told the judge, “I am going to plead guilty. I don’t plan to fight this.” Cullen said he did not want a lawyer and was held on $1 million bail.

Cullen, from Bethlehem, Pa., was charged with murder in the death of a Roman Catholic clergyman and the attempted murder of a 40-year-old woman at Somerset Medical Center. Investigators believe 12 to 15 of the deaths occurred at the hospital in Somerville.

The Very Rev. Florian J. Gall, vicar of Hunterdon County in the Roman Catholic Diocese of Metuchen, died June 28 at Somerset Medical Center. It was later determined that he had a lethal level of the drug digoxin, a heart medication, and had died from an unauthorized dose of the drug.

On June 16, the 40-year-old heart and cancer patient was discovered with an elevated level of digoxin. The woman recovered from the overdose and was released from the hospital, but died in September.

Somerset Medical Center said it fired Cullen Oct. 31 after an internal review found questionable lab results involving six of Cullen’s patients.

The lab findings prompted the hospital to notify prosecutors.