Ammonium nitrate has been used around the world in devastating terror attacks -- from Istanbul, Turkey to Oklahoma City.

There's even an al Qaeda video posted on the internet, showing how to mix ammonium nitrate to make a bomb.

"Ammonium nitrate is a weapon of choice for terrorists," said Congressman Peter King (R-NY), Chairman of the House Homeland Security Committee.

Yet as a widely used fertilizer, it is easily and cheaply purchased at farm supply and gardening stores around the country.

There is no federal law restricting, or even requiring registration of, who can buy it and no background checks required as we discovered in the course of an ABC News undercover investigation.

With virtually no questions asked, we were able to make large buys at four separate stores in Virginia and North Carolina.

A brochure put out by the fertilizer industry encourages farm stores to identify their customers by requiring a government-issued photo ID.

That did not happen at any of the stores we visited.

"When you're talking about something as lethal as ammonium nitrate, we have to have controls in place," Congressman King said.

But efforts in Congress to put in place background checks have failed, blocked by lobbyists for the American Farm Bureau as an unnecessary burden on farmers.

"Background checks would definitely be something that would cause many of the folks I work for a hard time," Rebeckah Adcock, Director of Congressional Relations at the American Farm Bureau, told ABC News.

As to whether a stranger could walk into an agricultural store with cash and buy ammonium nitrate, she replied, "I think that if he was in middle America, it would be unlikely."

Our ABC News investigation found otherwise.

In two days time, our undercover team, spending less than three hundred dollars, had assembled a thousand pounds of ammonium nitrate and, undetected, moved it into a storage shed just a few miles from the White House.

A thousand pounds would take down a good-sized building.

Congressman King says he's had to accept a watered-down version of his bill, with no background checks, in order to get even limited controls past the farm bureau lobbyists. The full House will vote on that bill sometime this fall.