When Tommy Thompson stood down as US health secretary in 2004, he delivered a stark warning.
Professor Larry Wein warned that milk could be an easy target
"I, for the life of me, cannot understand why the terrorists have not attacked our food supply, because it is so easy to do," he said.
Why was he so worried? Is "agro-terrorism" - attacking farming or the food supply - really so easy?
The only reported case in the US happened more than two decades ago in 1984, when a cult poisoned salad bars at a number of restaurants in Oregon. Forty people were taken to hospital, no-one died.
Mr Thompson had probably been listening to academics like Larry Wein, of Stanford University, who studies terrorist attacks that could kill more than 100,000 people.
Prof Wein found milk was particularly vulnerable to an attack. If someone were to put just 10 grams of botulinum toxin into a milk tanker, it could have devastating effects.
"If we didn't realise what was happening, half a million people would drink this milk... most of these would be poisoned, roughly half of them would die," he concluded.
Scary stuff, but critics said this was preposterous: obtaining even a tiny amount of toxin was a lot harder than Prof Wein suggested.
But the US government took his paper seriously. They called it a "road map for terrorists" and stopped its release. It was eventually published a month later.
If someone were going to target the food supply, Kansas would be an obvious place to start.
It is smack in the middle of the 100 mile-wide US beef belt, which produces 80% of the nation's beef.
Many of the cattle are raised on feed lots, huge expanses of land divided into fenced off dirt plots.
Mark Fisher runs one of these in Dodge City, where he farms about 18,000 cattle. This is industrial farming on a scale unheard of in Britain.
Mr Fisher is an active player in Agro Guard, a kind of "neighbourhood watch" for farmers.
They may be a hundred miles from the nearest big city, but he believes terrorism is a real worry.
"They could poison our feed stuff, water - I hate to even talk about it, we don't want to educate the terrorists," he says.
It is not just the farmers who are worried. Kansas State University is about to open a $50m Biosecurity Research Institute that will look at threats to agriculture - both natural and from extremists.
Dr Jerry Jaax, a former biological warfare specialist, has switched to studying agro-terrorism.
His biggest fear is a deliberate outbreak of foot and mouth disease, or FMD as they call it here.
"It doesn't have to be a terrorist - it could be a teenager with an attitude, a credit card and a passport," he says.
Kansas State University is bidding to be the home for a new federal bio- and agro-defence facility, a project that comes with $500m in government funding.
To secure such projects you need a big hitter in Washington - and Kansas has Senator Pat Roberts, chairman of the Senate Intelligence Committee.
He believes tackling agro-terrorism is part of the global war on terror.
Some farmers wonder whether the threat is being talked up to get a slice of the huge budget now spent on national security.
"Political influence has a tendency to take tax dollars and put it into certain areas that isn't for the good of society but makes somebody better off," says Donn Teske, president of the Kansas Farmers Union.
Mr Teske - who is a rare breed in the state, an organic farmer and a Democrat - believes the money going into protecting the food supply is a classic case of "pork-barrel" politics.
"Congressmen are notorious for that... they want to bring pork back to their area so they can be re-elected," he says.
Mr Teske may be sceptical, but agro-terrorism is not just the paranoia of a few farmers and academics. It is a big issue nationally.
Next month, Kansas City will host the second international conference on the subject.
The head of the FBI will be there, as will the US attorney general and senior law enforcement officials from the US and around the world.
Craig Watz, an FBI special agent who runs the agro-terrorism conference, says that when he talks at lunches or dinners, he emphasises how people need to change the way they think about food.
"How many people thought about the safety or security of food, who handled it, who prepared it, where it came from?" he asks.
"We do have to be vigilant not only getting on an airplane or in buses or train systems, but we also have to be vigilant in who's handling our food."
Before 11 September 2001, the US never thought about terrorism. Now it can't stop.
In Washington, analysts like John Feffer, of the magazine Foreign Policy in Focus, believes the government is scaremongering.
In the UK, foot and mouth led to the slaughter of 6.5m animals
"What does sell is a threat perception, fear is what sells, unfortunately that's what agro-terrorism feeds into," he says.
And what of the UK? The Americans I spoke to assumed that, having experienced foot and mouth in 2001, the British would be worried about agro-terrorism too.
When I called Defra, the UK's Department for Environment, Food and Rural Affairs, they could not find anyone who knew what it was.
After contacting the Home Office and London's Metropolitan Police, I ended up at the National Counter Terrorism Unit.
They told me: "We do not make public comments regarding specific threats, current intelligence or policy matters."
The security services do not appear to think attacking the food supply is a terrorist priority. Let us hope they are right.
The Silent Terrorist will be broadcast on Tuesday, 22 August 2006 on BBC Radio 4 at 2000 BST.
You can also listen online for seven days after that at Radio 4's Listen again page.