General Motors has sent a letter to Chevy Volt owners offering them a loaner car while the government investigates the safety of the Volt's battery.

The National Highway Traffic Safety Administration told GM late Thursday that another Volt battery caught on fire during testing, prompting the agency to open an investigation into the incidents. The agency was already looking into a fire that happened in a Wisconsin testing facility after a Volt underwent severe crash testing.

"Even though there have been no customer incidents, we're taking steps to ensure your peace of mind," wrote Mark Reuss, GM's president of North America, in a letter to Volt owners. "There is nothing more important to us at General Motors than the safety of our customers. We will continue to aid the NHTSA investigation in every way possible."

The Volt has become on of GM's most important cars, and not because they sell a ton (they sold just 5,000 this year through the end of October, out of 1.2 million in the U.S.) The car is vital to GM's image as a progressive company, one which isn't reliant on gas guzzlers and can introduce new technologies into the world.

The move to offer Volt owners alternative cars may seem like an overreaction, given the small number of incidents with battery fires. But overreacting gives GM the ability to tell customers the automaker puts their safety first – a message many automakers fail to deliver successfully. Many carmakers end up fighting with regulatory agencies to have their problems downplayed, which can result in a major public relations nightmare (a problem Toyota learned the hard way after suppressing accident data related to sudden acceleration crashes.)

The first fire happened in May, three weeks after a Volt was crashed for safety reasons. The coolant line ruptured, and NHTSA believes that was the cause of the accident.

Earlier this month, the agency tested three more batteries. Two did not catch fire. The third emitted sparks and smoke after it was intentionally damaged and then rotated 180 degrees. A week later, it caught fire.

"While it is too soon to tell whether the investigation will lead to a recall of any vehicles or parts, if NHTSA identifies an unreasonable risk to safety, the agency will take immediate action to notify consumers and ensure that GM communicates with current vehicle owners," NHTSA said in a statement.

Until its determined what caused the fires, NHTSA recommends that tow truck operators store the Volt in an open area, not a garage. Vehicle owners should also not store severely damaged vehicles in a garage, NHTSA said.

The agency is also looking into a house fire in Lake Norman, N.C., this month which happened when a Chevy Volt was charging inside the garage. Although it's unclear whether the fire started because of the Volt, because of its charging station, or because of a completely unrelated reason, Duke Energy told customers they should stop charging their electric cars in their home garages until further notice.

General Motors says an April fire in Barkhamsted, Conn., was not caused by the Chevy Volt charging in that garage, either. A 1987 Suzuki Samurai, which had been converted to run on electricity, was also charging in that garage.