MONROE — It starts with a sick cow. It continues with specially trained FBI agents walking through cow patties searching for evidence of a terrorism attack on a Monroe dairy farm.

It is what the FBI calls agroterrorism — when bad guys use viruses to attack the food supply instead of crashing airplanes into skyscrapers. It is a real threat and needs to taken seriously, experts said Thursday.

“We don’t want to get caught unprepared,” FBI Special Agent Peter de laCuesta said.

FBI agents joined National Guard troops, officials from the state Department of Agriculture and others Thursday for the first agroterrorism training exercise in the United States.

It happened on the Werkhoven Dairy Farm south of Monroe.

The drill was meant to bring officials from different government agencies together on a working farm to practice for a real attack.

“We need to help people know what to look for,” said Mark Kinsel, a state epidemiologist.

Biological agents, such as those that cause foot-and-mouth disease, are highly contagious, easily portable and could potentially devastate the economy, de laCuesta said.

From field to fork, one in six American jobs are tied to the food business, he said.

The beef industry alone contributes more than $182 billion to the U.S. economy, $3.6 billion in Washington, officials said.

“Bioterror is a major risk. As a cattleman, I’m extremely sensitive,” said Dale Reiner, a board member of the Snohomish County Farm Bureau. “All it takes is one cow.”

Thursday’s drill started with a simulated report of a Monroe cow infected with foot-and-mouth disease. In keeping with the scenario, state veterinarians started sounding the alarm while anti-terror experts in Washington, D.C., began gathering intelligence about a possible attack near Seattle.

As the training exercise played out, two men were arrested in a Seattle warehouse and teams were deployed throughout the region, including Monroe.

On Thursday, a SWAT team entered the Seattle warehouse, bomb-sniffing dogs searched Seattle’s Qwest Field and agents dressed in hazardous material outfits collected evidence at the Monroe farm.

“When I look around and see how susceptible we are to that kind of terrorism, I can’t imagine why (a drill) hasn’t happened sooner,” Reiner said.

Had an outbreak of foot-and-mouth disease really been discovered, every cloven-hoofed animal within a three-mile radius of the Monroe farm would have been quarantined and possibly euthanized, Kinsel said. Because of the highly contagious nature of the virus, other restrictions, including potential travel limitations, could also be imposed.

“It would require the destruction of much of the infrastructure,” Reiner said.

The virus causes blisters in infected animals’ mouths and on their hoofs, often causing lameness. It also reduces milk production in dairy cows.

Officials on Thursday underscored there’s no evidence of agroterrorism here.

Foot-and-mouth disease, which only very rarely infects humans, has devastated European farms and exists in South America, Africa and the Middle East, Kinsel said. There hasn’t been a case of the disease in the United States since 1929.

“We have a safe, secure food supply,” de laCuesta said.

That includes the 800 cows on the Werkhoven farm, which produces more than 7,500 gallons of milk each day, co-owner Andy Werkhoven said.

He welcomed the industry, agriculture and law enforcement officials invited to the farm to observe the exercise.

“It’s an excellent experience,” he said. “It’s a good opportunity for us to witness firsthand how we’re taking care of our food supply.”

It would be naive to think that threats don’t exist, Werkhoven said. Signs on his farm warn visitors from wandering through the barns where his black-and-white cows chomp feed.

The entire industry is vulnerable, he said. Still, there are systems in place in case something does go wrong.

“It’s one more tool that allows America to be great,” he said.