LAS VEGAS -- Sen. Harry Reid has called upon U.S. Interior Secretary Dirk Kempthorne to conduct a "full and thorough investigation" into the nitrate contamination deaths of 71 wild horses in July at the Tonopah Test Range.

"There are strong concerns in southern Nevada that these deaths are the result of serious negligence in the management of the test range and the wild horse herds in the area," Reid wrote in a letter sent Aug. 24 to the secretary.

"In light of this unfortunate event, I believe that this is also an appropriate time to take a close look at the land and wildlife management practices used on the larger Nellis Air Force Range," Reid, the Senate majority leader, suggested.

The Tonopah Test Range is within the Nellis Air Force Range boundaries.

The horses' carcasses were found at a watering hole about a mile from the Tonopah Test Range airfield.

High levels of nitrates were found in some water samples taken from a pond the horses used for drinking on a dry lake bed, the Bureau of Land Management reported Aug. 10, and also in the deceased horses' blood serum and ocular fluid.

Water tests indicated nitrate levels of greater than 3,000 parts per million (ppm). The federal standard for human consumption is less than 45 ppm. Livestock can tolerate higher levels, BLM reported, "but problems are known to occur when levels exceed 400 ppm, and acceptable levels should be below 100 ppm."

BLM manages the horse herds in the area while the Air Force oversees operations at the range.

According to a former Air Force technical sergeant who spoke to the Las Vegas Review-Journal recently, during the 1990s de-icing compounds high in nitrogen routinely ran off the airfield runway into the desert.

"I can't imagine that we can just turn a blind eye on this stuff being dumped illegally," Reid said. "I think it's something we should look at."

According to a BLM spokesperson, the bureau is developing an environmental testing strategy that includes additional water and soil sampling "to see if we can identify the source of the nitrates."

Acute nitrate toxicity is not well understood, and there haven't been many instances of it reported, stated a BLM spokeswoman earlier this month.

"There is no indication that the problem is attributable to a contagious or infectious disease," the spokeswoman said. "Tests conducted for salts, heavy metals and algae toxin have not identified other concerns."

She pointed out the deaths stopped when the water hole was fenced off.

Nonetheless, the source of the nitrates have not been clearly identified. BLM and the Air Force were expected to conduct further tests. Springs in the area will be analyzed as well. The barrier around the pound will be kept in place "until future testing shows the water quality has returned to safe levels for the horses," BLM reported.

The pond has been a source of water for at least 20 years, BLM said.

Capt. Justin McVay, a spokesman for Nellis Air Force Base, which uses the Nellis range for aircraft testing and air combat exercises, deferred comment to Secretary of Defense Robert Gates' office, saying it "would be inappropriate" for Nellis officials to speculate on a reaction to Reid's letter.

The Associated Press contributed to this article.