An unmarked police car pulls into a lawn-and-garden center on Long Island, N.Y., on a crisp spring day.

For Suffolk County Police Lt. James Rooney, it's a routine call in the name of homeland security. He asks the store to show him its supply of ammonium nitrate (search), but not because he feels there's anything suspicious.

"We reach out to them and kind of ask them ... if you see an anomaly in business practices, please give us a call," Rooney said. "For instance, if someone comes in, and you usually deal in a quantity of 10 pounds, and someone asks for 50 pounds."

A full 10 years after Timothy McVeigh (search) killed 168 people by mixing two tons of ammonium nitrate with fuel oil, putting it in the back of a truck and detonating the load in front of the Alfred P. Murrah Federal Building (search) in Oklahoma City, this potentially lethal fertilizer remains almost as widely available and is still largely unregulated.

In 47 states, anyone can walk in off the street and essentially need nothing more than cash to get ammonium nitrate. South Carolina, Nevada and Oklahoma are the only states that require buyers to show identification to buy the product, and in those states sellers keep records of those purchases.

Rep. Maurice Hinchey, D-N.Y., argues that federal legislation is the only way to ensure ammonium nitrate is used appropriately.

"You can regulate it in one state, but if the adjoining state or some other state doesn't regulate it, you can easily transport it by truck across our state without anyone knowing it," Hinchey said.

The upstate New York congressman has introduced the Ammonium Nitrate Security Act (search) in the House of Representatives. The bill would require anyone storing, buying or selling the fertilizer to obtain a federal permit, one which might require a federal background check.

"This is a material you just can't have unregulated on the open market, because it is so potentially dangerous and it is so readily available," Hinchey said. "Anybody can walk into a place where this is sold and buy as much as they want under the present set of circumstances."

In 1998, the National Academy of Sciences (search) recommended that if terror threats were to increase nationwide, Congress should respond appropriately with some form of ammonium nitrate regulation.

After the Oklahoma City bombing, the fertilizer industry launched a voluntary campaign to keep track of ammonium nitrate.

"We have done an excellent job in the states of implementing voluntary measures to ensure that the product stays safe," said Kathy Mathers, vice president of public affairs at the Fertilizer Institute (search).

Hinchey countered: "I do commend the industry for the steps it's taken, and I think they are positive steps but they do not nearly go far enough. They leave an awful lot of big wide loopholes open."

Almost by proxy, rural America has become the unofficial security guard of the nation's fertilizer stockpile.

In one rural county in northeastern Missouri, only a padlock guards a pile of ammonium nitrate.

Distributor Brad Patterson said ammonium nitrate may be largely unregulated, but that doesn't mean he's not vigilant. He said he only sells to regular customers and won't let strangers make purchases.

Last year, more than one a half million tons of ammonium nitrate were sold in the United States, according to the Fertilizer Institute.

But in light of the mounting Al Qaeda threats, the farm and fertilizer industries, which had opposed federal regulatory efforts, changed their minds.

"We as an industry are absolutely committed to security and ensuring that our product is not used in another Oklahoma-type incident," Mathers said.

But 10 years later, as the nation remembers those killed in the Murrah Federal Building, a chief ingredient in McVeigh's bomb remains widely available.

"I don't know why it hasn't been regulated," Hinchey said. "I was very surprised, shocked to find out that it wasn't, but it really must be. It must be regulated. It must be regulated approrpriately, so that we can reduce the likelihood of people who are going to use it for destructive purposes."

Click in the video box for a report by FOX News' Jeff Goldblatt.