Bernadette Protti went on trial before a judge in March 1985. By then 16 years old, the teen's taped confession took center stage. The victim's family took exception to statements that Protti made about Costas' alleged use of marijuana while they sat in her car, one of the issues that Protti said initiated their argument. "I just didn't want to, and she made me feel dumb," Protti said in her confession (via People).

The prosecution sought a first-degree murder conviction but Judge Edward Merrill said at the end of the three-day trial that they failed to prove there had been premeditation. He convicted Protti of the lesser offense of second-degree murder, per The San Francisco Examiner. "My feeling is that the law has been served," Arthur Costas said after the trial. "I'm not in agreement with the punishment, but that's my opinion. I don't believe the punishment could ever match the crime in this case. We've lost our daughter," Merrill later gave Protti a nine-year sentence to be served at a juvenile facility. At the sentencing hearing, Costas' mother, Berit, gave an emotional statement, telling the judge (via the Oakland Tribune), "Kirsten's hopes and ideals were shattered on June 23, 1984. She was attacked by Bernadette Protti and left in the street to die." At the time, the case made national headlines and eventually became fodder for television.

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