Usually radioactive dust thrown into the atmosphere by above-ground atomic bomb testing, predominantly in the 50s, and which was banned by a treaty in the 60s. Also the result of underground testing gone wrong.

Fallout was dispersed widely across the US by winds as the result of it's test program; the most concentrated fallout was in Idaho, hundreds of miles away from Nevada. The US govt denied causing sheep deaths in Utah, until a court concluded otherwise in the 80s.