In February 1998, Cullen was hired by the Liberty Nursing and Rehabilitation Center in Allentown, Pennsylvania, where he staffed a ward of respirator-dependent patients. There, Cullen was accused of giving patients drugs at unscheduled times. He was fired after being seen entering a patient's room with syringes in his hand—an encounter that left the patient with a broken arm, but without criminal injections. Cullen caused a patient's death at Liberty Hospital, which was blamed on another nurse. After leaving Liberty, Cullen was employed at Easton Hospital in Easton, Pennsylvania, from November 1998 to March 1999. On December 30, 1998, he murdered yet another patient with digoxin. A coroner's blood test showed lethal amounts of digoxin in the patient's blood, but an internal investigation within Easton Hospital was inconclusive; evidence did not definitively point to Cullen as the murderer.