DETROIT (AP) - US safety regulators are investigating complaints that the rear suspension frames can rust and cause failures on Hyundai Sonata mid-sized cars.

The probe affects about 393,000 cars from the 2006 through 2008 model years.

The National Highway Traffic Safety Administration says it has received six complaints of suspension failures with three happening at highway speeds.

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 but two of the failures activated the cars' electronic stability control systems.

The agency says investigators will figure out if the problem is bad enough to cause a recall.

A message was left seeking comment from a Hyundai spokesman.