Knowing the ATF they probably thought they were Mexican drug cartels. Is this confirmed, however? Maybe one was loosely tied to some criminal from the distant past that doesn't matter. Or just and ego stroke for the special agent in charge. If anything, it's the always tyrannical faggots, stupidity knows no bounds.
However, there is a serious possibility with the arizonian-federal conflict that they are trying to disarm the arizonians as a "quelling" measure.