WASHINGTON (AP) — The watchdog for spending in Afghanistan is warning commanders that a safety system to protect U.S. troops from roadside bombs is incomplete or inadequate.

In a letter released Thursday, Special Inspector General for Afghanistan Reconstruction John Sopko told top generals that an Afghan contractor who was supposed to install 125 of the systems along strategic highways left the work undone or poorly done.

The systems are largely metal grates, put over the openings of culverts that go under highways so insurgents can’t plant improvised explosive devices, or IEDs.

“Given the increased risk of IED attacks against U.S. forces resulting from missing or defective … systems, we are providing this information to you for immediate action and dissemination to all relevant personnel,” Sopko’s safety alert said.

It was sent Wednesday to the top commander in Afghanistan, Gen. John Allen, and commander for the Middle East, and Gen. James N. Mattis, head of the U.S. Central Command.

Sopko said he found the “potentially significant contract fraud” while looking into a $362,000 contract signed in 2011. Because a criminal investigation is under way, the company’s name is not being released, said Philip J. LaVelle, spokesman for the inspector general’s office.

He also declined to comment on whether any troop deaths have been caused by the problem with the culvert grates.

It wasn’t immediately known how many other contractors have been paid to do similar work in other locations around the country.