Actually, the people in charge are saying this:
Belmar stressed that the approximately six people involved in the exchange of fire that led to the shooting by police should not be associated with the demonstrations. “They weren’t protesters; they were criminals,” he said.
source: the Guardian http://www.theguardian.com/us-news/2015/aug/10/ferguson-shooting-gunfire-heard-during-anniversary-rally
This has revealed a major fault, which is that plainclothes police do not carry any form of bodycam or dashcam, which could tell us what actually happened. I am also disturbed that while they say the protesters and the criminals are separate groups, they have decided to use chemical warfare on the protestors.
“Failure to disperse may result in your detention, arrest, and/or exposure to chemical or less-lethal munitions,” a police officer said over a loudspeaker. “Police intend to use chemical munitions.” While some were swept down the main street others were shot by a bombardment despite retreating down the side street as requested by police.
Again, the Guardian, same article. Usually a reasonably trustworthy source, when they can figure out how to spell.