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The U.S. beef industry’s dependence on the muscle-building drug Zilmax began unraveling here, on a sweltering summer day, in the dusty cattle pens outside a Tyson Foods Inc slaughterhouse in southeastern Washington state.

As cattle trailers that had traveled up to four hours in 95-degree heat began to unload, 15 heifers and steers hobbled down the ramps on August 5, barely able to walk. The reason: The animals had lost their hooves, according to U.S. Department of Agriculture documents reviewed by Reuters. The documents show the 15 animals were destroyed.

The next day, the hottest day of the month, two more animals with missing hooves arrived by truck. Again, the animals were destroyed, the documents show.

The animals’ feet were “basically coming apart,” said Keith Belk, a professor of animal science at Colorado State University. Belk said he reviewed photos of the lame cattle, though he declined to say who showed them to him.

The 17 animals had a factor in common, according to an examination of U.S. government documents and interviews with people who had direct knowledge of the events. In the weeks before the cattle were shipped to Tyson’s slaughterhouse, outside the city limits of Pasco, all had been fed Merck & Co Inc’s profit-enhancing animal feed additive, Zilmax.

The day after the hoofless animals were euthanized on August 6, Tyson told its feedlot customers it would stop accepting Zilmax-fed cattle. After Reuters reported the existence of a videotape of apparently lame Zilmax-fed animals – shown by an official of meatpacking giant JBS USA LLC at a trade meeting in Colorado – Merck itself temporarily suspended sales of the drug in the U.S. and Canada. The rest of the nation’s leading meatpackers soon followed Tyson, the largest U.S. meat processor.

Merck, in a statement to Reuters, stressed the safety of its product. It said the company investigates all reports of adverse reactions to its drugs, and did so after the deaths near Pasco.

“Several third-party experts were brought in to evaluate the situation, review the data and identify potential causes for the hoof issue,” Merck’s statement said. “The findings from the investigation showed that the hoof loss was not due to the fact these animals had received Zilmax.”

Merck declined to identify the names of the third-party investigators or provide more detail on the research findings.

After temporarily halting Zilmax sales, Merck continues to state Zilmax is safe when used as directed, with no welfare concerns discovered in 30 research studies since the product was introduced in the United States in 2007. In addition, Merck said, the company is planning more field evaluations of Zilmax, using “a well-designed collection and analysis of data by third-party industry experts.” A “prominent” epidemiologist and veterinarian will oversee the work, Merck said.