The murders that became the first to which Cullen later confessed, occurred at St. Barnabas. On June 11, 1988, Cullen administered a lethal overdose of intravenous medication to a patient. Cullen eventually admitted killing several other patients at St. Barnabas, including an AIDS patient who died after being given an overdose of insulin. Cullen left St. Barnabas in January 1992 when hospital authorities began investigating who had contaminated IV bags. That investigation determined that Cullen had most likely been responsible, resulting in dozens of patient deaths at the hospital.