Who killed Katie in the movie Mystic river?

Spoiler Alert! Katie's boyfriend's brother, Ray Harris Jr. and his friend John killed Katie because he found out Brendan and Katie were planning to elope in Las Vegas. Mystic River is a 2003 drama film directed and co-produced by Clint Eastwood and filmed on location in Boston, Massachusetts.

The film stars Sean Penn, Tim Robbins, Kevin Bacon, and Laurence Fishburne and was based on a novel of the same name. Mystic River was nominated for Academy Awards for Best Picture, Best Director, Best Adapted Screenplay, Best Actor, Best Supporting Actress and Best Supporting Actor. Mystic River was the first film, since Ben-Hur in 1959, to win Best Actor and Best Supporting Actor.

Mystic River was well received by critics and viewers, with the movie earning $156 million worldwide, which is significantly higher than its $30 million budget.