There are many interesting details in this article on the eroding separation between the US military and civilian police and law enforcement agencies. It ties together several trends and events that I knew of but didn't see the connections. From Bush's Posse Roundup:

Over the past three years, the Bush administation has accelerated a trend of using the military as a tool in our nation’s domestic affairs. From its support of the Total Information Awareness surveillance vacuum cleaner, to its use of Pentagon spy planes during the Washington-area sniper shootings in late 2002, to its attempt to empower military officials to seize Americans' financial and other private information without a warrant, the Bush administration gives grave cause for concern about the growing role of the armed forces in our daily life.

Unfortunately for our democracy, in recent decades, the restrictions on using the military at home have been eviscerated, particularly under this sitting president. And because the Bush administration is so intent on secrecy, and because the Congress during Bush’s presidency has almost totally defaulted on its duty to conduct oversight, we have little idea of how often the Posse Comitatus law is now being violated. The few Bush efforts that have become public do not inspire confidence.

The Patriot Act of 2001 created a new Information Office in the Pentagon that promptly launched work on the Total Information Awareness (TIA) system, which was a project of the Defense Advanced Research Projects Agency (DARPA). TIA was intended to create a massive dragnet to build dossiers on American citizens—seeking “connections between transactions – such as passports, visas, work permits, driver’s licenses, credit cards, airline tickets, rental cars, gun purchases, chemical purchases – and events such as arrest or suspicious activities and so forth," according to Undersecretary of Defense Pete Aldridge.

Another major assault on Posse Comitatus occurred when two snipers went on a rampage in the Washington, D.C. area in October 2002. The Bush administration quickly called in Pentagon spy planes to canvas the entire Washington area. The use of the RC-7 planes, operated by military personnel, appeared to be a brazen breach of the Posse Comitatus Act. But the mass panic that gripped the Washington area indicated how feeble the status of Posse Comitatus is. In the political world after 9/11, laws appear to provide far less restraint on the use of the military than in the past.

The military planes provided no information that aided the apprehension of the suspects. Instead, they epitomized how a massive federal-state-local response did nothing to compensate for a shortage of street smarts and common sense detective work. The response by local governments and the FBI to the sniper rampage was one of the biggest Keystone Kops episodes in recent U.S. history: the FBI had ignored five different people who contacted them months before the shootings to warn about John Allen Muhammad's homicidal comments, local police ignored eyewitness reports about the snipers' Chevrolet Caprice at the scene of shootings, and the snipers' auto passed through at least five police roadblocks erected after their attacks. Eventually the two suspects were caught in routine law enforcement fashion. The military’s efforts went for naught —they only signalled the Bush adminstration’s willingness to apply military force to the domestic populace.