DETROIT (AP) — U.S. safety regulators are investigating about 218,000 Mercedes C-Class luxury cars because the rear lights can fail and even catch fire.

The National Highway Traffic Safety Administration says the probe affects cars from the 2008 and 2009 model years.

The agency says it has 21 reports of the brake lights or turn signals lights dimming or failing to light. In many cases the drivers reported a burning smell or melting of electrical parts. Some also reported smelling smoke or seeing burn marks in the trunks, and one reported small flames coming from the rear lamp connector when the trunk was opened.

The problem hasn't caused any injuries. It normally happens in either the left or right brake or tail lights, but in some cases both sides have failed to work, NHTSA said in documents posted Monday on its website. The agency said it opened the investigation on July 11.

Investigators will look into whether the problem is widespread enough to warrant a recall.

A Mercedes spokesman said Monday that he was checking into the matter.

In one complaint filed with NHTSA from February of this year, an owner said that while driving at night, the dashboard warning lights came on indicating tail light failures on both sides of the car. The driver pulled over and opened the trunk to find it full of smoke with the smell of burning plastic. The owner told NHTSA that both bulb-holding assemblies and the electrical connections had been heavily damaged by fire and heat and said the cars should be recalled.

In another case, from June of 2012, a C-Class owner told NHTSA that the right rear tail light wiring harness caught fire and melted. The owner wrote that a dealer in Springfield, Mo., was contacted, and the owner was told that Mercedes would not fix the problem under recall. "This is a tremendous safety issue and I respectfully request that your agency address this matter with Mercedes," the owner wrote.

People who file complaints are not identified by NHTSA.