http://www.msnbc.msn.com/id/5906750/At agricultural co-ops across America, it's available by the truckload. Farmers spread more than two million tons of it across the U.S. every year.
It's ammonium nitrate, one of the world's cheapest fertilizers. But it can also be a powerful explosive when mixed with ordinary diesel fuel.
Timothy McVeigh used 4,000 pounds of that mixture to rip apart the Oklahoma City federal building. It's also a terror weapon of choice for making car and truck bombs — like the one in Bali, Indonesia in 2002, and in Turkey twice last November. In March 2004, British police found half a ton of it while rounding up suspected terrorists.
Now, with new warnings that al-Qaida planned to hit U.S. financial targets using car and truck bombs, some in Congress are calling for tighter restrictions on ammonium nitrate.