The U.S. military has had some recent success intercepting CAN crossing the border from Pakistan into Afghanistan.
This summer, U.S. troops discovered 5,000 pounds of Pakarab calcium ammonium nitrate hidden beneath a floorboard of a truck, still in original bags. But they admit they still don't know enough about the networks that move the fertilizer or just how large they are.
"It's like narcotics crossing the U.S. border," said the senior U.S. military intelligence official. "We don't know if we're getting 1 percent or 10 percent that's crossing." The U.S. has also had some success targeting bombmakers.
Two senior military officials cited the case of a raid in Afghanistan's Paktika province that killed 11 militants. IEDs did not return to the immediate area, the officials said, for seven months.
When pressed, however, U.S. officials concede that the porous border, weak Pakistani law enforcement and the relatively small amount of CAN required to make bombs mean the flow of bomb ingredients might never be completely stopped.
"There is no silver bullet to fix this problem," said the U.S. official in Pakistan, a phrase repeated almost exactly by three of the U.S. officials interviewed for this story.
Military advocates of counterinsurgency believe that ultimately, the problem might have to be tackled inside Afghanistan, by using Afghans themselves as the bomb detectors.
In Kandahar, for example, U.S. military officials say that after a surge of U.S. troops removed insurgents from traditional safe havens, the number of weapons and materiel caches reported to them by the local population spiked by 400 percent.
"The tipping point is preventing attacks and giving confidence to the population," a senior U.S. military official said. "They become your defense."
But the U.S. government has clearly decided to try and make pressuring Pakistan to limit the CAN crossing the border one of its top priorities -- even if all that is possible in the end is to make CAN-based IEDs more expensive and difficult to make.
"CAN has a legitimate use and banning isn't a realistic strategy in Pakistan, where agriculture is so important," says Casey. "But if we can just make it a little harder for the guy building the bomb, it's worth our time."