In the wake of Sept. 11, the U.S. government spent years, and billions of dollars, fortifying various industries against possible terrorist attacks. And since then, the United States has seen its fair share of terrorist attacks, including the April 2013 Boston Marathon bombing.

While the food system has remained relatively untouched, "we've certainly studied it since 9/11 to assess what the potential impacts might be," says Don Kraemer, deputy director of the FDA's Center for Food Safety and Applied Nutrition. "And they can be catastrophic," he says.

The FDA rules focus on weak links in food processing and manufacturing in an attempt to ferret out where the vulnerabilities exist.

The rules mostly apply to facilities in charge of bulk storage or handling of liquids for human consumption – think dairy plants where milk is stored in big vats. Another area of concern? Facilities like large, industrial bakeries where lots of ingredients are mixed together.

"A lot of food processing manufacturers don't practice rigid biosecurity," says Peter Chalk, a terrorism analyst with the RAND Corporation, a policy think tank.

Many food companies fail to take even the most basic precaution, he says. Owners don't padlock warehouses or engage in sufficient surveillance. Or they hire a lot of transient workers without performing background checks.

"So actually, introducing a contaminant — salmonella, botulism, mercury — into the food chain would not be particularly difficult," Chalk says.

The weak links, though, haven't really been tested. The last big bioterror attack in the U.S. happened in 1984 in The Dalles, Ore. That's when members of a cult infected salad bars with salmonella; more than 700 people were sickened. Since then, the American food system has grappled more with unintentional outbreaks, like the listeria-laden cantaloupe that killed 33 people in 2011.

Would the FDA's proposed rules keep us safe? Chalk says the vulnerabilities go well beyond what's covered in the proposal.

Producers could be at risk as well, he says. It would be relatively easy to deal a devastating blow to the country's livestock industry with a virus in a vial. An act of agroterrorism like that keeps some food experts up at night.

If a terrorist wanted to deal a devastating economic blow to the U.S., all it would take is a calculated release of foot-and-mouth disease on the nation's livestock. Unintentional outbreaks in Europe and South America have haunted economies there, as trade is shut down and whole herds are culled to quarantine the disease.

The impact of a deliberate outbreak in the U.S. could be huge. One risk assessment from the Department of Homeland Security found that if a pathogen like foot-and-mouth disease were let loose among Great Plains ranchers, total damages could exceed $50 billion. Exports and trade could be cut off, and consumer demand would likely take a huge hit.

When U.S. troops raided an al-Qaida storehouse in Afghanistan in 2002, they found documents detailing ways to attack American agriculture in order to deal a blow to the U.S. economy. Still, no attack has materialized in the 12 years since.

"Agriculture is critical infrastructure in a country," says Keith Roehr, Colorado's state veterinarian. "How would we eradicate the disease? We don't know. ... We know there would be steps we would take. Do we know exactly what these would be? No, we don't."

Yet few livestock owners consider their operations targets of terrorism, Roehr says. And that mindset could leave them vulnerable.

Experts suspect that the bigger reason the U.S. has avoided a large-scale attack on food and farms is that an attack like that doesn't carry the same weight as a suicide bombing or mass shooting.

"It lacks a visible point for the media to latch onto, [except for] the possible images of burning cows," Chalk says. "Really, it doesn't have the same blood lust appeal of carrying out a suicide attack in a shopping mall."

Still, it's a risk that government regulators want the food industry to consider more seriously, in case what seems like a grisly fiction turns into reality.

A version of this post first appeared on the Harvest Public Media website.

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Could Our Food Supply Be A Target For Terrorists?