HAZMAT Alert! That white powder on the ground or pavement is probably NOT an anthrax attack. Although everyone must be on alert, there is a high probability that what appears to be handfuls of white bread flour (including markings with circles or X's), is part of a trail left by one of the many hundreds of Hash House Harriers and Hounds jogging groups in America or overseas.
Check the local Hash trail schedule for your area. If they were laying a trail within a couple of miles of your mysterious white powder, then it's probably flour.