How? Take a look at the laundry list of charges, on 6 people, and you tell me how in a little over 2 weeks that they went through a thorough investigation of the situation. The medical report came out just yesterday, the ink is not even dry yet.
The most time-consuming part of any investigation is identifying, tracking down and interviewing witnesses. Since the vast majority of this incident happened in the police transport van, the universe of witnesses was quite small.
Given that I'm sure whatever team they had investigating this matter was given all the resources they need, I'm not surprised they finished the investigation so soon after they received the medical report. I've been part of much more complex investigations that took just as little time - throw enough people at an investigation, and things can move very quickly.
They likely had a working theory of what happened, based on their inteviews of the officers, other transportee(s), and any others, and they really just needed to see if the medical report was consistent with their findings. Once the medical report came in and was consistent with their findings, it was just a matter of dotting their i's and crossing their t's, so to speak.