In the eyes of her teen-age classmates, Kirsten Costas, 15, had it all -- a varsity cheerleader, athlete, always surrounded by friends.

That prized popularity may have been the motive for Kirsten's murder, perhaps at the hand of a jealous classmate still in school, her father and several students suggest.

Breaking a 2 1/2-month silence, Arthur and Barit Costas talked to reporters about their daughter's stabbing death at the hands of a teen-age girl described by witnesses as a chunky blonde. "I feel a group of girls was involved . . . to set this up," Costas said. "Then it got out of hand."

According to authorities, Barit Costas received a call June 21 from a girl who said she would pick Kirsten up for a school function. About an hour after being picked up, deputies said, Kirsten appeared at a house in nearby Moraga asking to use the telephone to call her parents because her friend was "acting weird." Her parents were not home, and so a man in the house drove her to her neighbor's house.

But a female teen-age driver followed them, the man said, and when Kirsten got to her neighbor's front porch, the girl got out of her own car, slashed Kirsten several times and fled.