WASHINGTON (AP) - Federal officials say they are investigating the safety of the lithium-ion battery in General Motors Co.'s Chevrolet Volt after a second battery fire following crash-testing of the electric car.

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The National Highway Traffic Safety Administration said Friday that three Volt battery packs were crash-tested last week. In one instance, the battery caught fire afterward, and in another the battery emitted smoke and sparks.

Last May, a fire erupted in the battery of a Chevy Volt that had been damaged during a government crash test three weeks earlier. Last week's tests were an attempt to replicate the May fire.

NHTSA has opened a formal safety defect investigation of the batteries.

General Motors officials said previously that government officials didn't follow the carmaker's protocols for storing post-crash batteries.