what forensic proof? do you have access to the investigators notes?
There are many problems that law enforcement as a whole is facing in this case
Chief among them is that people - normal people from everyday life - now have access to all the fancy software and computers that used to be only available to those who had money.
And more and more people are not only educated about these matters, they are able to communicate instantaneously around the world.
And finally, nearly everyone has a video and audio recording device on their person - or close by - 24/7/365. So incidents like this one are more often than not recorded by many different people from many different locations and vantage points.
It isn’t difficult for an independent person - or lab - to choose one of these videos and perform a forensic audio analysis on small portions of the recording (to minimize any chance of operator movement), use the video to find an approximate location for the recording device using Google Maps, and then analyze the audio and come up with actual physical evidence that there either was or wasn’t more than one shooter.
It’s simple physics and geometry - and the software to pull out and listen to the audio at whatever speed you need is not only cheap, but my mom could probably figure out how to work it. She may not know how to calculate the time/speed/distance or know what sound is what - but she could work the software and I could step her through the rest...
So the bottom line is that for many aspects of any “investigation” some members of the public no longer need to wait for those in authority to tell them what happened - they can begin to piece it together themselves.
Shorter answer to your question: The analysis didn’t have to be done by the “official” investigators therefore access to their information isn’t necessary.