<P> Katie goes out for the night with her girlfriends and Dave sees her at a local bar . That night, Katie is murdered, and Dave comes home with an injured hand and blood on his clothes, which his wife Celeste helps him clean up . Dave claims he fought off a mugger, "bashed his head into the concrete", and possibly killed him . Sean, now a detective with the Massachusetts State Police, investigates Katie's murder . His pregnant wife, Lauren, has recently left him . </P> <P> Over the course of the film, Sean and his partner, Sergeant Whitey Powers, track down leads while Jimmy conducts his own investigation using his neighborhood connections . Sean discovers that the gun used to kill Katie was also used in a liquor store robbery during the 1980s by "Just Ray" Harris, the father of Katie's boyfriend . Harris has been missing since 1989, but Brendan claims he still sends his family $500 every month . Brendan also feigns ignorance about Ray's gun but Sean believes it was still in the house . Sergeant Powers suspects Dave as a possible perpetrator because he was one of the last people to see Katie alive . He also has a wounded hand and, although he continues to tell his wife he got it while being mugged, he tells the police a different story--soon Jimmy becomes suspicious of it . Dave continues to behave strangely, which upsets his wife to the point she is afraid he will hurt her . While Jimmy and his associates conduct their investigation, Dave's wife eventually tells Jimmy about Dave's behavior, the bloody clothing, and her suspicions . </P> <P> Jimmy and his friends get Dave drunk at a local bar . When Dave leaves the bar, the men follow him out . Jimmy tells Dave that he shot "Just Ray" Harris at that same location for ratting him out and sending him to jail . Jimmy informs Dave that his wife thinks he murdered Katie and tells Dave he will let him live if he confesses . Dave repeatedly tells Jimmy that he did kill someone but it was not Katie: he beat a child molester to death after finding him having sex with a child prostitute in a car . Jimmy does not believe Dave's claim and threatens him with a knife . When Dave finally admits to killing Katie thinking he can escape with his life, Jimmy kills him and disposes of his body in the adjacent Mystic River . </P> <P> While Dave is being killed, Brendan (having found out about his father's gun during questioning) confronts his younger brother Ray Jr. and his brother's friend John about Katie's murder . He beats the two boys and threatens to kill them if they do not admit their guilt, but when John takes the gun and is about to shoot him, Sean and Powers arrive just in time to stop it . </P>

Who killed katie in the movie mystic river