they say they recovered the gun at the scene. they say he fired THREE shots, which is a specific number (rather than just vaguely saying "shots were fired"), so they might have already verified that.

with people already out protesting on the street as soon as it happened, they can't just sit back while they wait on the results of a GSR test, they have to report those first two pieces of information before people's minds get set that this was another Mike Brown.