Kirsten Costas

Protti claimed to have found the kitchen knife by chance, and her elder sister, Virginia Varela, testified in court that she kept that knife in her car to cut vegetables. The Costases did not believe Protti's story – they asserted that nobody would use a butcher knife to slice tomatoes and that Protti, casually dressed on that evening, never intended to take Kirsten to a party but had planned to murder her. Protti was sentenced to a maximum of nine years, but was released seven years later on parole.

In 1994, the story was made into a television movie entitled A Friend to Die For (also known as Death of a Cheerleader), with Tori Spelling as Stacy Lockwood, a character based on Kirsten Costas and Kellie Martin as Angela Delvecchio, a character based on Bernadette Protti. It was remade by Lifetime as a 2019 television movie, starring Aubrey Peeples as Bridget Moretti and Sarah Dugdale as Kelli Locke playing characters based on Protti and Costas, respectively, with Kellie Martin also appearing. Costas' murder was featured in Season 5 of Deadly Women. The Kirsten Costas murder was the subject of Season 1, Episode 3 of Investigation Discovery's The 1980's: The Deadliest Decade.

American filmmaker James Benning covered the aftermath of the murder in his 1987 documentary Landscape Suicide.

The Costas family left Orinda and moved to Hawaii in 1986. Bernadette Protti was released from prison in 1992 at the age of 23 and reportedly left California, and changed her name to Jeannette.

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 family's 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 butcher knife and fled. The Costas' neighbors called an ambulance, but Kirsten was mortally wounded and died at a nearby hospital.

Kirsten Marina Costas (July 23, 1968 – June 23, 1984) was an American high school student who was murdered by her classmate, Bernadette Protti, in June 1984.