It is documented that the U.S. Army intentionally tested biological weapons on American civilians, mostly in the 50s and 60s. From The New York Times archive in a 1994 article:

The Army released an organism called Aspergillus fumigatus at the Norfolk Naval Supply Center because most workers were black; for some reason, the testers imagined an enemy might target the blacks at military bases. “Since Negroes are more susceptible to coccidioides than are whites,” a report said, “this fungus disease was simulated by using Aspergillus fumigatus.” Aspergillus, further, was known to cause lethal infections.

In 1949, and for 20 years afterward, the Army released bacteria among millions of unsuspecting people. At hearings in 1977, Pentagon witnesses acknowledged that bacteria and chemical particles were sprayed over San Francisco, St. Louis, Minneapolis and 236 other populated locations.

The Army conceded that it had released microorganisms at Washington National Airport in 1965 and into the New York City subway system in 1966 during peak travel hours. The purpose was to see how the bacteria spread and survived as people went about their routine activities.

New York Times

The article goes on to say that the Army still (as of 1994) intentionally tests microorganisms at Dugway Proving Ground in Utah. Chemical weapons were also once tested in the same area, with devastating results for nearby sheep.