COLORADO SPRINGS, Colo. (AP) – About 30 cattle died in Colorado last month after ingesting high levels of nitrates on a rancher’s property near Schriever Air Force Base, a veterinarian said.

Dr. John Heikkila, of Choice Veterinary Service, examined the animals and said that elevated levels of nitrates were found in at least one bale of hay eaten by the cattle, The Gazette reported.

The rancher, who was not identified, fed the cattle the same hay for two weeks without incident before their deaths on Dec. 22, Heikkila said.

Nitrates can be found in some livestock feeds and are common in fertilizers and explosives. At high levels, it can lower the oxygen-carrying capacity of animal blood, resulting in death, he said.

Heikkila said a hay broker who sold the feed to the rancher is testing additional bales for high levels of nitrates. No additional cattle deaths were reported.

El Paso County sheriff’s deputies investigated the incident last month and determined there was no criminal activity involved, authorities said.

Schriever Air Force Base is located about 20 miles (32 kilometers) east of Colorado Springs.