(November 28, 2011) The National Highway Traffic Safety Administration is investigating fires involving the lithium-ion batteries in General Motors' Chevrolet Volt after another fire last week.

One Volt battery pack that was being closely monitored following a government crash test caught fire Thursday, the NHTSA said in a statement Friday. Another recently crash-tested battery emitted smoke and sparks, the statement said.

GM said in a statement that the Volt "is safe and does not present undue risk as part of normal operation or immediately after a severe crash."

The latest fires are in addition to a battery fire in a crash-tested Volt six months ago.

"Because of these test results, NHTSA has opened this investigation to examine the potential risks involved from intrusion damage to the battery in the Chevrolet Volt," the agency said.

In the latest NHTSA tests, the agency intentionally damaged the battery compartment and ruptured the vehicle's coolant line to replicate the initial incident. Whereas the first fire occurred weeks after a crash, the latest incidents happened within hours or days of the crash, it said.

The Volt uses a lithium-ion battery to power the car, along with a small gas engine that kicks on when the battery runs low. Auto makers, as they bring electric cars and gas-electric hybrids to market, are using lithium-ion batteries rather than nickel-hydride systems used in popular hybrids like Toyota's Prius. NHTSA expects about 70 percent of battery-powered cars will feature lithium-ion batteries in the next decade.

In a change from previous statements, NHTSA is no longer saying it believes battery-powered cars to be as safe as traditional vehicles. It is continuing a broader review of electric-car safety that was focusing on the effect of crashes on batteries.

Sources: Media reports