On June 23, 1984, Costas was lured with a phony invitation to a dinner for the Bob-o-Links, a sorority-like group at school. According to Protti's later testimony, she had planned to take Costas to the party to befriend her, but Costas got angry when she was told that there was no dinner for the new "Bobbies". The girls quarreled, and Costas fled to the home of Alex and Mary Jane Arnold, living nearby, telling them that her friend had gone "weird". When Costas could not reach her parents by telephone, Alex Arnold drove her home, noticing that a Pinto–the Protti's family car–was following them. At the Costas home, Arnold, sitting in his car, saw Protti attack Costas. He thought that he was seeing a fist-fight but, in fact, Protti stabbed Costas five times with a kitchen knife and fled. The Costas' neighbors called an ambulance, but Kirsten was mortally wounded and died at a nearby hospital.