They do not want to panic the public. Hence, ignoring the 800 pound gorilla in the room (foreign terrorism of a crack urban guerilla nature right within our borders) that officials in MD, VA, DC and feds would eerily seem to be purposely steering clear of in talking with the press or other announcements.
No doubt the FBI has seen the pages of the urban terrorist training manuals seized in Afghanistan, with a concentration on urban attacks including sniping at soft targets, and immediate extraction, to reappear at another spot and do the deed again, only to disapper. Multiple, repeated, random--all designed to produce panic. (Which the US capital area is starting to approach as the panic virus is being spread through kids to parents, among co-workers and within the ranks of everyday commuters).