Federal officials say they believe nitrate contamination killed 71 wild horses last month along a dry lake bed near a desert airfield.
Officials say they're still investigating what caused the contamination near the Tonopah Test Range.
Nitrate levels in the animals were at least 66 times the safe drinking standards for humans and 30 times acceptable levels for livestock.
The Tonopah range is a high desert airfield 210 miles northwest of Las Vegas.
It's used for commuter aircraft that shuttle workers from Las Vegas to remote sites where tests of stealth aircraft and other high-tech aircraft have been conducted.
One former worker at the test site told the Las Vegas Review-Journal that he believes the contamination came from illegal dumping of chemicals used in the de-icing of planes and a nearby runway.