Charles Graebar: Someone was spiking IV bags with insulin in the store room.
Charles Graeber, a New York writer as well as a former medical student and researcher, has spent seven years investigating Cullen's murders for a new book called "The Good Nurse." Graeber says a number of patients at St. Barnabas went into insulin shock and nearly died.
Charles Graeber: He was the main suspect for poisoning random bags of saline. If you talked to the investigators there, they'll tell you, "Cullen was our man. We knew he was dirty." They couldn't prove anything. It's all circumstantial.
Steve Kroft: They fire him?
Charles Graeber: He moved on.
When Cullen left the hospital, the insulin overdoses stopped.
Charles Cullen: At St. Barnabas, they could've had my license investigated and probably revoked at that point in time.
Steve Kroft: Should they have?
Charles Cullen: Should they have? Yes.
But instead of ending Charles Cullen's nursing career, St. Barnabas marked the beginning of a 16-year killing spree.
Cullen would work at eight other hospitals and be suspected of harming patients at six of them, but those suspicions never reached subsequent employers and Cullen continued to murder patients with virtual impunity. In 1993, prosecutors investigated Cullen for murdering 91-year-old Helen Dean. An autopsy tested for nearly 100 medications, but not the one Cullen used to kill her: a powerful drug called digoxin, or dig for short. It was Cullen's first weapon of choice.
Steve Kroft: Why did you like dig?
Charles Cullen: Dig, you know, it was a very powerful cardiac medication.
Steve Kroft: What does it do to someone?
Charles Cullen: In small amounts, it slows the heart rate down. In larger amounts, it can cause what's called complete heart block. And then, the heart is very irregular. And you know, it can cause death. It does cause death, in large amounts.
It was also readily available in critical care units, and Cullen figured out ways to conceal his digoxin withdrawals from an automatic drug dispensary system called Pyxis, which required nurses to type in the name of the patient and the drug to be administered.
Charles Cullen: I wouldn't go in for dig. I would go under Tylenol or another medication that would be in the same drawer. /So, you know, there was no record of me going in for dig other than the fact that, you know, it was in the same drawer.
Steve Kroft: How did you choose who you're going to give this medication to?
Charles Cullen: It's difficult for me to go back in time and think about what things were running through my mind at the time.
Steve Kroft: Was it personal?
Charles Cullen: No, no.
Steve Kroft: Did you get pleasure out of it? Satisfaction? Relief?
Charles Cullen: No, I thought that people weren't suffering anymore. So, in a sense, I thought I was helping.