Hundreds of residents of Clarksville, Tennessee town have been told to stay indoors and turn off their air conditioners after small, ultra-light planes dropped a mysterious, powdery substance over 2 subdivisions there on Thursday evening.

The unknown substance was dropped on about 180 homes. Initial Tests have reported that it is not anthrax, and further testing will be done to try to determine what it is.

Clarksville is near Fort Campbell, Kentucky, home of the 101st Airborne Division. The parkway near the base was shut down at about 9 p.m. The Red Cross has set up services in a nearby elementary school, in case they are needed.

Strangely enough, this is the second time a mysterious powder has turned up in the area within the week. The day before, on Wednesday, the North Clarksville Gas and Water Service Center was shut when a white, powdery substance was found there by a janitor. The city is testing this as well.

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