The assumption was that the gangs were young black males since the media early on latched on to the narrative that New Orleans before, during, and in the immediate aftermath of Katrina was being held siege by desperate, criminal, bands of out of control black thugs. Despite all evidence to the contrary that debunked this lie, it was looped so long and so often it became accepted fact.
It still is because it rests on the pantheon of stereotypes and negative typecasting of young black males and Williams likely lie had deadly consequences in New Orleans with the number of young black males that were assaulted and in some cases killed by police during the Katrina chaos. Put plainly, it's the shortest of short steps to think that if innocents can be depicted as a caricature of the terrifying image that much of the public harbors about young black males, then that image seems real, even more terrifying, and the consequences have been just as lethal consequences for other black males.