Kathryn, your Army LTC is incorrect. The Posse Comitatus Act is not a bar to the use of the military for domestic law enforcement. It merely ensures that the military can be used only when directed by the president or when Congress authorizes it, not at the whim of a federal marshal.
Contrary to what many Americans believe, the Constitution itself does not prohibit the use of the military in domestic affairs. Indeed, the U.S. military has intervened in domestic affairs some 167 times since the founding of the Republic. In 1807, at the behest of President Thomas Jefferson, who was troubled by his inability to use the regular Army as well as the militia to deal with the Burr Conspiracy of 1806-07, Congress also declared the Army to be an enforcer of federal laws, not only as a separate force, but as a part of the posse comitatus.
Accordingly, troops were often used in the antebellum period to enforce the fugitive slave laws and suppress domestic violence. The Fugitive Slave Act of 1850 permitted federal marshals to call on the posse comitatus to aid in returning a slave to his owner. In 1854, Franklin Pierce’s attorney general, Caleb Cushing, issued an opinion that included the Army in the posse comitatus, writing that
A marshal of the United States, when opposed in the execution of his duty, by unlawful combinations, has authority to summon the entire able-bodied force of his precinct, as a posse comitatus. The authority comprehends not only bystanders and other citizens generally, but any and all organized armed forces, whether militia of the states, or officers, soldiers, sailors, and marines of the United States.
But the use of soldiers as a posse removed them from their own chain of command and placed them in the uncomfortable position of taking orders from local authorities who had an interest in the disputes that provoked the unrest in the first place. As a result, many officers came to believe that the involvement of the Army in domestic policing was corrupting the institution. Thus the Army welcomed the Posse Comitatus Act (Section 1385, Title 18 U.S.C.), which prohibits the use of the military to aid civil authorities, in enforcing the law or suppressing civil disturbances except in cases and under circumstances expressly authorized by the Constitution or Act of Congress. As Robert W. Coakley, one of the foremost authorities on the use of the military in domestic affairs, has written:
All that [the Posse Comitatus Act] really did was to repeal a doctrine whose only substantial foundation was an opinion by an attorney general [Caleb Cushing], and one that had never been tested in the courts. The president’s power to use both regulars and militia remained undisturbed by the Posse Comitatus Act. . . . But the Posse Comitatus Act did mean that troops could not be used on any lesser authority than that of the president and he must issue a “cease and desist” proclamation before he did so. Commanders in the field would no longer have any discretion, but must wait for orders from Washington [emphasis added].
I wrote something on this for NRO back in Oct 2005.