At least three dairy cows have died after being exposed to a toxic substance in Washington state, and the Food and Drug Administration, the Department of Homeland Security and the FBI have begun investigations into who and what caused their deaths.

In a release issued last night, the FDA said that an unnamed toxic substance had caused at least three deaths on a farm in Enumclaw, about 50 miles southeast of Seattle. The incident was first reported to law enforcement on June 6 but was made public yesterday.

The release said that FDA and FBI investigators visited the farm over the weekend and yesterday and that tissue samples from a cow that died Saturday were sent to the FDA's forensic chemical center, a specialized laboratory in Cincinnati.

"It could be a prank. It could be a criminal offense. It could be someone trying to pollute the food supply," said Raymond G. Lauer, spokesman for the FBI office in Seattle. "It might be something and it might be nothing. At this point, we just don't know."

According to the FDA, no milk from the exposed animals has entered the food supply. It said that "fewer than 20" dairy cattle were involved in the incident, and that some of the exposed animals did not become ill. The agency, as well as Lauer, said the incident appeared to be isolated to one farm and one herd.

The collection of agencies that have converged on the farm indicates that criminal activity is suspected. The FDA Office of Criminal Investigations is involved, as well as the sheriff's office for King County, Wash., the FBI, and the Homeland Security Department. The U.S. Department of Agriculture and the Washington State Department of Agriculture have also been brought in, according to the FDA release.

The FDA, which is leading the investigation, said it will provide more specific information when it becomes available.

Enumclaw is a small town due east of Tacoma that has Mount Rainier as a backdrop. The town is about 100 miles northwest of Mabton, the site of the dairy farm that housed the first animal in the United States identified to have mad cow disease.