For the first time, investigators are revealing how they were finally able to bring an end to Charles Cullen's shocking killing spree -- along with the identity of the informant who ultimately cracked the case.

Cullen, who was working as a nurse at Somerset Medical Center in New Jersey at the time of his arrest, is currently serving 17 life sentences for confessing to 40 murders in New Jersey and Pennsylvania, although detectives believe he may have killed as many as 400. But the details of how police were finally able to catch Cullen, who was working as a nurse at Somerset Medical Center in New Jersey at the time of his arrest, have remained a secret until now.