General Motors is contacting every owner of a Chevrolet Volt to assure them the extended-range electric car is safe and allay fears it could catch fire after a crash.

In addition, GM is going to give any owner who still has concerns another GM car while the federal investigation of Volt is underway. The offer came on a conference call with reporters this morning.

The National Highway Traffic Safety Administration announced Friday that it has opened a formal investigation into the Volt. It said it was taking the action after several tests in which the car's lithium-ion battery heated up or caught fire after a severe crash. But the fire didn't break out immediately. In some cases, it took weeks, leading to warnings for tow operators and storage yards to make sure the cars are parked away from buildings or other vehicles. No injuries or fires in customers' vehicles have been reported.

"We are contacting all Volt owners to assure them and reassure them the car is safe to drive," said Mark Reuss, head of General Motors' North America. "The Volt is our pride and joy, and we will do all we can to make Volt owners as happy with it as we can." Reuss says letters to 5,329 owners with the offer are going out starting today.

The Volt is a plug-in car that runs on electricity for 25 miles or more. Then its gas engine kicks in to recharge the battery. GM says it believes the car's electronics may be at fault, not the advanced battery itself. In total, GM has made more than 11,000 of the $39,995 cars. It was North American Car of the Year this year and is considered breakthrough technology.