DETROIT - The government ended its safety investigation into the Chevrolet Volt on Friday after concluding that the Volt and other electric cars don't pose a greater fire risk than gasoline-powered cars.

The agency began studying the Volt last June after a fire broke out in one of the cars three weeks after it was crashed as part of safety testing. Two other fires occurred later related to separate safety tests, and NHTSA opened an official investigation into the vehicle on Nov. 25.

The agency and General Motors Co. know of no fires in real-world crashes.

GM and federal safety officials believe the fires were caused by coolant leaking from damaged plastic casing around the batteries after side-impact collisions. The coolant caused an electrical short, which sparked battery fires seven days to three weeks after the crashes.

GM announced earlier this month that it will add steel plates to approximately 12,000 existing Volts to protect the batteries in the event of a crash. The company has sold around 8,000 Volts and 4,000 are still for sale. GM is repairing the vehicles for free. NHTSA didn't order the recall, as it sometimes does after a safety investigation.

GM said Friday that NHTSA's decision to close the investigation is consistent with the results of its own internal testing. It said the steel plates will provide additional protection and minimize fire risk in the days and weeks after a crash.

NHTSA said Friday that it "continues to believe that electric vehicles show great promise as a safe and fuel-efficient option," and that based on available data, electric cars don't appear to be riskier than gas-powered ones.

But the agency said electric cars do have some specialized components, and the agency has developed guidelines for firefighters and other responders on how to handle electric cars after a crash.