Three weeks after a side-impact crash test on May 12, the Volt caught fire while parked in the NHTSA testing center

U.S. government safety regulators are conducting an investigation into the safety of lithium batteries in plug-in electric vehicles as a result of a Chevrolet Volt fire earlier this year.

Back in May, General Motor Co.'s Chevrolet Volt underwent a series of tests at the National Highway Traffic Safety Administration (NHTSA) facility in Wisconsin. Three weeks after a side-impact crash test on May 12, the Volt caught fire while parked in the testing center.

The fire was serious enough to burn other vehicles parked nearby, according to sources close to the talks who have requested to stay anonymous because the investigation is not yet public.

The fire has prompted an investigation of the safety of lithium batteries used in plug-in electric vehicles such as the Volt and Nissan's Leaf. Lithium batteries can catch fire if the internal cells or the battery case are pierced by steel or another ferrous metal, making the batteries a potential problem in car crashes. However, the batteries are usually heavily protected to prevent this from occurring.

Nevertheless, the NHTSA is looking into the safety because U.S. President Barack Obama is looking to put 1 million electric vehicles on the roads by 2015, and if there are any issues with these batteries, it wants to find it sooner rather than later.

Regulators have requested information about lithium batteries used from GM, Nissan, and Ford, and others who currently sell or plan to sell electric vehicles in the future.

GM defended the Volt after the fire, saying that the EV's battery, which is supplied by LG Chem Ltd., pose no greater threat than conventional cars. It added that GM has certain safety procedures for the Volt and the handling of its battery after an accident, and if these procedures would have been followed, the fire wouldn't have occurred.

"There are safety procedures for conventional cars," said Greg Martin, GM spokesman. "As we develop new technology, we need to ensure that safety protocols match the technology."

Even though the fire occurred three weeks after the side-impact crash, Munro said a small piercing of the battery can lead to a reaction days or weeks later.

The Wisconsin fire, however, is not the only EV fire that has occurred recently. NHTSA also sent investigators to Mooresville, North Carolina after a residential garage, which contained a charging Volt, caught fire. The investigation is ongoing.

"As manufacturers continue to develop vehicles of any kind -- electric, gasoline or diesel -- it is critical that they take the necessary steps to ensure the safety of drivers and first responders both during and after a crash," said NHTSA in an email statement on Friday. "Based on the available data, NHTSA does not believe the Volt or other electric vehicles are at a greater risk of fire than gasoline-powered vehicles. In fact, all vehicles -- both electric and gasoline-powered -- have some risk of fire in the event of a serious crash."

Katherine Zachary, spokeswoman for Nissan's U.S. unit, added that the Nissan Leaf hasn't had any reports of a fire. Over 8,000 Leafs are on U.S. roads today.

"The Nissan Leaf battery pack has been designed with multiple safety systems in place to help ensure its safety in the real world," said Zachary. "All of our systems have been thoroughly tested to ensure real-world performance."