On March 13, 1968, a malfunctioning spray nozzle on an F-4 Phantom carrying 320 gallons of VX nerve agent turned a classified test at Utah's Dugway Proving Ground into a disaster. Residual chemical drifted into neighboring Skull Valley and killed thousands of sheep.
The Army denied responsibility, settled with ranchers and buried the carcasses without admitting fault. Its own investigators reached a far more definitive conclusion, but the report stayed classified and out of public reach for three decades. The incident permanently ended open-air chemical weapons testing in the United States.
Dugway and the Testing Program
The Army established Dugway Proving Ground in 1942 on roughly 800,000 acres of Utah's west desert, about 85 miles southwest of Salt Lake City. Germany and Japan were both suspected of developing chemical weapons, and the service needed a testing site removed from any populated area.
By 1958, Dugway housed the Army Chemical, Biological, and Radiological Weapons School. Testing through the following decade covered artillery shells, aerial bombs and spray systems built to distribute agents across defined zones. Most activity was classified, and the majority of Americans had no knowledge the program existed.
In the years before 1968, Army personnel conducted at least 1,100 documented chemical tests at the facility. VX was among the agents under development.
Categorized as a weapon of mass destruction under United Nations conventions, VX is a contact agent designed to fall to the ground and persist rather than disperse as a gas. A dose as small as 10 milligrams absorbed through the skin can kill.
The agent works by blocking the enzyme that allows muscles to relax, triggering uncontrollable contractions that eventually prevent the respiratory system from functioning.
Three Tests on March 13
Army personnel at Dugway conducted three open-air operations involving nerve agents on March 13, 1968. The first was a test-firing of a chemical artillery shell. The second involved burning 160 gallons of nerve agent in an open pit.
For the third, an F-4 Phantom flew with two TMU-28B spray tanks, each loaded with 160 gallons of VX. The mission required dispersal over a target area 27 miles west of Skull Valley, then a climb out of the drop zone.
One tank did not empty fully during the pass.
As the jet ascended after completing its run, residual VX continued leaking from the dispenser and trailed into a higher atmospheric column. Wind carried the chemical northeast, across the mountain range separating the proving ground from the ranches beyond. It reached Skull Valley before the next morning.
The Aftermath of the Tests
Tooele County Sheriff Fay Gillette drove into Skull Valley that morning after ranchers reported animals dying across the range. The sight stayed with him for years.
"I've never seen such a sight in my life," Gillette later told investigative reporter Seymour Hersh.
"It was like a movie version of 'death and destruction' — you know, like after the bomb goes off. Sheep laying all over. All of them down — patches of white as far as you could see."
The surviving animals displayed a consistent pattern. Researcher Philip Boffey later documented that they were "generally act[ing] dazed, [with] their heads tilted down and off to the side, walk[ing] in a stilted, uncoordinated manner."
Those symptoms matched what toxicologists expected from organophosphate nerve agent exposure, which locks muscles in contraction by blocking the enzyme that normally allows them to release.
Scientists from the National Communicable Disease Center in Atlanta collected water, forage grass, blood and liver tissue from the area. Their analysis found the compounds recovered were "identical" to Army-supplied VX and could "only be attributed to the same chemical."
When the die-off ended, roughly 6,400 sheep were dead. Another 600 animals too damaged to survive were slaughtered. Many hundreds more were potentially contaminated, preventing them from being sold for profit.
Denial and Acknowledgement
Dugway officials told reporters in the days after the deaths surfaced that no chemical weapons testing had been conducted in the relevant period. That stance lasted one week. On March 21, 1968, Utah Sen. Frank Moss, a Democrat, released a Pentagon document confirming the March 13 VX spray operation.
The Army shifted its stance but did not acknowledge fault. Brig. Gen. William W. Stone led a formal investigation that generated more than 1,000 pages of findings.
That report acknowledged probable traces of nerve agent in environmental samples from the kill zone. A compound found in sheep tissue bore a direct relationship to Dugway's own VX stockpile. The Army's formal position remained that it had not been negligent and accepted no legal liability.
Quietly, the service paid rancher Alvin Hatch, whose animals made up roughly 90 percent of the dead, a settlement of $376,685. Military bulldozers helped bury the carcasses on base property.
Rancher Ray Peck, whose sheep also died, later reported that members of his family developed nervous system ailments consistent with low-level VX exposure. Follow-up analysis found that medical evaluations the Army had used to argue no humans were harmed were inconclusive by later scientific standards.
The Cover Up
Despite national press coverage, political accountability within Utah was muted. The military was the state's largest employer, and elected officials prioritized economic continuity over pressing for answers.
Hersh, who investigated the incident, identified the likely reason. "Concern, from the highest level of state officialdom on down, was that too much investigating or talking about the incident might make the Army move its base from Dugway," Hersh reported.
The Army managed the fallout rather efficiently. It settled with the primary rancher, arranged burial of the carcasses and initiated a safety review, all without conceding fault. State officials had enough cover to move on. The story's national reach came from outside Utah entirely.
On Feb. 4, 1969, NBC's newsmagazine program "First Tuesday" aired a segment on the Skull Valley deaths. Rep. Richard McCarthy, a Democrat from New York, watched it and was troubled by what he learned.
McCarthy had believed, as most Americans did, that chemical weapons had been effectively barred under international agreements since 1925.
McCarthy and the Congressional Investigation
"Chemical and biological weapons were another side of the nuclear arms race, but they were a much more secret and hidden aspect of it," science historian Roger Eardley-Pryor later told Smithsonian Magazine. "They were much less known until Richard McCarthy made this a national issue."
Beginning in May 1969, McCarthy opened congressional hearings that revealed the scale of what had been occurring without public knowledge. The program responsible for disposing of chemical weapons carried the acronym CHASE, which stood for Cut Holes And Sink 'Em.
The hearings disclosed that hundreds of thousands of tons of old munitions had been loaded onto ships and dumped at sea with minimal documentation of quantity or location.
Later that year, a nerve gas leak at a U.S. military installation on Okinawa injured 24 people. The press connected that accident to the Utah deaths, and the Pentagon confirmed that open-air nerve agent testing had also occurred at Edgewood Arsenal in Maryland and Fort McClellan in Alabama.
The scandal became impossible for the military to ignore.
Nixon Acts
Facing compounding pressure from Congress and the public, President Richard Nixon signed an executive order in 1969 banning all open-air testing of chemical and biological weapons. He separately renounced the U.S. biological weapons program entirely.
Dugway's research budget fell by 60 percent and the Army Chemical Corps came close to being disbanded altogether. Congress followed in November 1969 with legislation formally prohibiting open-air testing of any lethal chemical or biological warfare agent within the United States.
Live-agent testing at domestic facilities was restricted to secure laboratories and chemical weapons development effectively stopped. Nixon also submitted the 1925 Geneva Protocol to the Senate for ratification, a step the United States had declined to take for 44 years. The Senate ratified it in 1974.
The Classified Report
The Army commissioned an internal study on the sheep kill that was classified in 1969. It was declassified in 1978 but remained unpublicized for another two decades, sitting in government files without reaching a general audience.
In 1998, the Salt Lake Tribune obtained the document. Its central finding was described at the time of publication as the "first documented admission" from the Army that a nerve agent had caused the Skull Valley deaths. The word the report used was "incontrovertible."
Even after the document became public, the Army declined to formally acknowledge negligence. The Chemical Weapons Convention of 1993 codified broader international prohibitions that public pressure over Skull Valley had helped push into existence.
A malfunctioning spray nozzle and 6,400 dead sheep in the Utah desert accomplished more toward constraining America's chemical weapons program than four decades of international diplomacy had managed before them.
Sources: Smithsonian Magazine, "How the Death of 6,000 Sheep Spurred the American Debate on Chemical Weapons," Lorraine Boissoneault, April 2018; Modern War Institute at West Point, "Who Killed the Dugway Sheep? Why It Matters Fifty Years Later," March 2018; Science, Philip M. Boffey, "Nerve Gas: Dugway Accident Linked to Utah Sheep Kill," Dec. 27, 1968; Utah State Historical Society, History to Go, "Chemical Weapons Testing Created Controversy at Dugway"; Interesting Engineering, "How a Nerve Agent Killed Thousands of Sheep and Transformed Weapons Law in the US," Dec. 2021; Axios Salt Lake City, "Flashback: When Nerve Gas Killed Thousands of Utah Sheep Near Dugway," March 2024; Brig. Gen. William W. Stone Army investigation, 1968; National Communicable Disease Center findings on sheep tissue samples, 1968.
