U.S. safety regulators are investigating complaints that the rear suspension frames can rust and fail on Hyundai Sonata midsize cars.

The probe affects about 393,000 cars from the 2006 through 2008 model years. The National Highway Traffic Safety Administration say it has received six complaints of suspension failures, with three happening at highway speeds, according to documents posted on its website on Friday. The investigation was opened on March 25.

The agency says rust in the frame can cause control arm failures. A car's wheel hubs are attached to the control arms. No crashes or injuries were reported.

- Associated Press