President Obama has repeatedly said his top counterterrorism goal is to prevent terrorists from acquiring the building blocks to make nuclear or “dirty” bombs. In April of 2009, Obama announced a new international effort to “secure all vulnerable nuclear material around the world within four years.” Since then, the Department of Energy has dispatched scientists around the globe to collect hundreds of pounds of the stuff.

But according to a report from the Government Accountability Office (GAO), issued late last Friday afternoon to little fanfare, thousands of pounds of highly-enriched uranium and separated plutonium remain. American officials may never get a chance to ensure its security.

That’s because the U.S. can’t track or fully account for 5,900 pounds of “weapons usable” nuclear material that it once shipped overseas. Instead, U.S. officials have to rely on foreign governments’ assurances that the potentially cataclysmic stuff is safe. And when those officials occasionally visit the sites holding the nuclear material, nearly half the places “did not meet International Atomic Energy Agency security guidelines,” according to the GAO, Congress’ investigative arm.

“It’s amazing how completely cavalier the Department of Energy has been at tracking this. They’ve got nobody who worries about this on a day-to-day basis,” says Jeffrey Lewis, a nuclear weapons analyst at the Monterey Institute of International Studies (and occasional contributor to this blog).