DETROIT (AP) — U.S. regulators have closed an investigation into steering failures on some Ford Super Duty trucks, saying they didn't find a safety defect.

The National Highway Traffic Safety Administration opened the probe in May after getting complaints of steering gear box failures in F250 and F350 trucks. The investigation covered about 340,000 trucks from the 2008 model year.

The agency said Tuesday that lab tests of failed parts found no evidence of fatigue. Also, an analysis of complaint rates showed no pattern of manufacturing quality problems.

The agency says the complaint rate for the trucks is low. It says that in many complaints, the trucks had hit curbs and other objects, suggesting that crashes caused steering failures.

Ford and the agency received 149 complaints about the problem. No injuries were reported.