In August, Reid asked the Interior and Defense departments to probe the horse deaths. The carcasses were found in July at the Tonopah Test Range, 210 miles northwest of Las Vegas.

An Aug. 10 statement from the BLM noted that high levels of nitrates were found in some water samples taken from a pond that the horses used for drinking on a dry lake bed. The levels were at least 66 times in excess of safe drinking standards for humans and 30 times in excess of acceptable levels for livestock.

Some former workers at the Tonopah Test Range and another installation in the Nellis Air Force Range complex have said de-icing fluids and pellets high in nitrogen content were routinely used and released uncontrolled into the environment at remote airstrips during the 1980s and 1990s.

In his letter to Reid, Wynne acknowledged that elevated nitrate levels were found "in a pond in a Sandia National Laboratory test area." He noted that Sandia uses the Tonopah Test Range under an Air Force permit with the National Nuclear Security Administration.

Sandia dug a pond for a test program in the Tonopah Test Range main lake playa "over 20 years ago. In August, the multi-agency response team fenced off the excavated hole to safeguard the 1,000 remaining horses," Wynne's letter reads.