Possible Causes
Previously obtained documents said one such demonstration also occurred the day before the mysterious sheep deaths in 1968. On 13 March 1968, an F-4 Phantom strike aircraft flew a test mission over the Dugway Proving Ground with chemical dispensers containing VX. One of the dispensers wasn't completely emptied during the test, and as the F-4 gained altitude after its bombing run, VX trickled out in a trail behind the aircraft, drifted into Skull Valley, north of the proving ground, and settled over a huge flock of sheep.
One explanation in the aftermath of the incident was that a chemical or biological agent had escaped from the Dugway Proving Ground. Circumstantial evidence seemed to support this assertion, the United States Army admitted to conducting open-air tests with the nerve agent VX in the days preceding the sheep kill. The Army intimated that a spray nozzle had malfunctioned during the test causing an aircraft to continue spraying VX as it climbed to higher altitudes. It was reported that a small amount of VX was found in the tissue of the dead sheep.
Other information contradicted the initial assumptions about the cause of the incident. One contradiction to nerve agent exposure as a cause came in the symptoms of some of the sheep following the incident. Several sheep, still alive, sat unmoving on the ground. The sheep refused to eat, but exhibited normal breathing patterns and showed signs of internal hemorrhaging. Regular breathing and internal hemorrhaging are inconsistent with nerve agent exposure. In addition, no other animals in the area, some much more susceptible to nerve agent poisoning, were affected.
Read more about this topic: Dugway Sheep Incident