Tactical officers fire tear gas on Aug. 11 in Ferguson, Mo.

Photograph by Robert Cohen/St. Louis Post-Dispatch via AP Photo

Tactical officers fire tear gas on Aug. 11 in Ferguson, Mo.

Photograph by Robert Cohen/St. Louis Post-Dispatch via AP Photo

Military-Style Police Forces

Tactical officers fire tear gas on Aug. 11 in Ferguson, Mo.

Photograph by Robert Cohen/St. Louis Post-Dispatch via AP Photo

A SWAT robot, a remote-controlled small tank-like vehicle with a shield for officers, is demonstrated for the media on April 18, 2013 in Sanford, Maine 

Photograph by Robert F. Bukaty/AP Photo

Police officers gather next to the Waukesha Sheriff Department's BearCat on Aug. 8, 2013 in Elm Grove, Wisc.

Photograph by Abe Van Dyke/Corbis

Police in riot gear watch protesters on Aug. 13 in Ferguson, Mo. 

Photograph by Jeff Roberson/AP Photo

A police officer watches over demonstrators protesting the shooting death of teenager Michael Brown on Aug. 13 in Ferguson, Mo.

Photograph by Scott Olson/Getty Images