From: http://publicintelligence.net/dodd-macdis/
Quote4.1.1. The President is authorized by the Constitution and laws of the United States to employ the Armed Forces of the United States to suppress insurrections, rebellions, and domestic violence under various conditions and circumstances. Planning and preparedness by the Federal Government and the Department of Defense for civil disturbances are important due to the potential severity of the consequences of such events for the Nation and the population.
More at the link.
This is from 1994!