I agree that they could do a better job, but I think you are making a mistake thinking that there is no need to keep the investigation confidential. People are speculating that there may have been other shooters. I don’t think that there were, but that is something the police are surely going to investigate, if only to rule it out.
Now, what if they DO find evidence of another shooter? If they had gone off half-cocked and released all the evidence to the media before discovering that, they would have tainted their investigation. So, it’s better to err on the side of caution, even if you think the perp is dead and there will be no trial.
Still, some things they surely could release, but haven’t, and that is disappointing.