In Mystic River, How did listening to the 911 tape solve the case?

Sean and Whitey knew enough facts about the case to at least make a weak connection to Silent Ray and John O'Shea after listening to the tape.

1. They assumed that Katie knew her killer because the old lady witness told them that she heard a "Hi" followed by the shot.


2. They knew that the gun used to kill Katie was the gun (not just caliber or type, but THE gun) that Just Ray used in a liquor store robbery years ago based on the ballistic analysis.


3. Katie was killed in the Harris' neighborhood, assumedly on her way to Brendan's house (they were leaving for Vegas the next day.)


4. You can tell from the call that they were kids and they knew that the victim was a girl but were obviously trying to hide what they knew.

So all of this adds up to (in Sean and Whitey's mind): How can all the evidence point to Brendan and yet, based on the polygraph, he is not the killer? Answer: They were looking at the wrong brother. That at least warrants a conversation with the boys. The rest is explained by Qrious above.

As I stated in another post, from a writer's perspective, the call is simply the mechanism that allows the characters to break the case. Whether the call is a plausible case-breaker or whether the call itself is realistic seems to be the question. On the former, yes, on the latter, that remains open for debate.