In one test, a hopper car full of fertilizer yielded an explosion equal to 20,000 pounds of TNT, a senior Defense official said. That is about the equivalent of the largest conventional ordnance in the Pentagon's arsenal, and four times the estimated size of the Oklahoma City blast.
Detonating ammonium nitrate requires a booster, such as widely available special gunpowder or harder-to-get plastic explosives.
Mixing fuel oil with ammonium nitrate is sometimes done to improve its explosive power but is not necessary.
The Fertilizer Institute asserts that there is no public evidence that fertilizer with a concentration of nitrogen lower than 34% can be easily detonated. "I would like to see the data on that," said Kathy Mathers, its vice president for public affairs. "ATF has refused to share the testing data."
An ATF spokeswoman said Friday that she had no information on the issue and she could find no official available to respond.
At least some of the test data have been classified by the Defense Department. But current and former ATF officials -- who spoke with the Los Angeles Times on condition of anonymity because they were not authorized to discuss the matter publicly -- said Pentagon tests showed conclusively that lower concentrations could detonate, and that videos of those tests were shown to the Fertilizer Institute staff.
Critics of the ammonium nitrate act also say the legislation splits jurisdiction over explosives between Homeland Security and the ATF.
After the Sept. 11, 2001, terrorist attacks, the ATF was given responsibility for tight new regulations on explosives such as TNT and dynamite under the Safe Explosives Act of 2002.
"We have made policy recommendations that ammonium nitrate be regulated by ATF, so as not to create one more agency with jurisdiction," said Jeffrey L. Dean, executive director of the International Society of Explosives Engineers, a technical group based in Cleveland.
"The more agencies that get involved, the more confusion that exists, and the more confusion that exists, the less safety we have."
The explosives industry has worked closely with the ATF to improve domestic control of explosives, Dean said, a system that was difficult to implement but is now working well. Currently, federal licenses are required to purchase dynamite and TNT, as well as some prepared blends of ammonium nitrate and fuel oil, but not raw ammonium nitrate.
Outside groups are asking for tougher action. "Congress simply didn't understand what it was doing," said Peter Stockton, senior investigator for one of the groups, the Project on Government Oversight, which is a watchdog on national security issues.
"Maybe they thought doing something was better than nothing."