Contract fraud in Afghanistan has yielded a major unexpected threat to U.S. troops from homemade bombs, according to the U.S.’ contracting watchdog in the country. And the watchdog agency considers the threat so dire that it took the rare step of tipping its hand on an active criminal investigation to warn the military command in Afghanistan.

John Sopko, the Special Inspector General for Afghanistan Reconstruction, is warning that “potentially significant contract fraud” from a $361,680 deal with an Afghan company may have led troops to falsely believe that metal bars placed over water drainage systems and roadside holes are effective against implanted insurgent bombs. A letter Sopko sent to Marine Gens. John Allen, commander of U.S. troops in Afghanistan and James Mattis, commander of U.S. forces in the Mideast and South Asia, says a “large number” of the so-called “culvert denial systems” were “not installed or were installed in a defective manner.”