In a development that might prove to be a serious setback for electric vehicles, the Chevrolet Volt plug-in hybrid vehicle from General Motors Co. (GM) is being subject to a formal safety defect investigation because of two fire incidents resulting during crash tests.

With a Volt having caught fire this summer nearly three weeks after a side-impact crash test, the National Highway Traffic Safety Administration (NHTSA) launched an investigation on Friday to determine whether a damaged Volt is a potential fire hazard.

The investigation chiefly comes after crash tests on a number of Volts and their batteries resulted in fires; with one of the instances - at a test facility in Burlington, Wis. - being so dangerous that the fire in one of the test vehicles engulfed three other vehicles parked close by.

The key concern that the NHTSA has voiced against the Volt is that the tests - which had been so designed that they replicated real-world collisions - revealed that the lithium-ion batteries of the Volt got badly damaged during a crash and caused the vehicles to catch fire.

However, despite having initiated investigations into the potential fire instances linked to the Volt, the NHTSA has also stated that while 5,000 Volts have been sold thus far, there have been no reports of fires for any of the vehicles involved in a highway crash.

Meanwhile, about the federal investigation that has got underway recently, GM said that the Volt "is safe and does not present undue risk as part of normal operation or immediately after a severe crash."