By Alisa Priddle, USA TODAY and the Detroit Free Press

The federal safety investigation into post-crash fires in Chevrolet Volt batteries formally closed today. The car got a clean bill of health.

The National Highway Traffic Safety Administration posted its defect investigations results late Friday saying the investigation, which opened Nov. 25, is now officially closed.

NHTSA released a statement, analysis of its battery testing, fire incident reports and guidance for emergency responders late Friday in announcing the conclusion of its investigation.

The statement read: "The agency's investigation has concluded that no discernible defect trend exists and that the vehicle modifications recently developed by General Motors reduce the potential for battery intrusion resulting from side impacts.

"Based on the available data, NHTSA does not believe that Chevy Volts or other electric vehicles pose a greater risk of fire than gasoline-powered vehicles," NHTSA said, noting all vehicles have some risk of fire in the event of a serious crash.

NHTSA believes General Motors' efforts to retrofit Volts in dealerships and on the assembly line addresses safety concerns. The Hamtramck, Mich., plant has been on an extended holiday shutdown but will resume production Feb. 1, said GM spokesman Greg Martin.

GM spokesman Greg Martin said of the NHTSA announcement: "NHTSA's decision to close their investigation is consistent with the results of our internal testing and assessment.

"The engineering enhancements that GM announced Jan. 5, 2012, will provide additional protection for the battery minimizing the risk of a post-crash fire in the days and weeks after a severe crash and rollover."

The modifications include extra steel around the battery pack to prevent vehicle parts from puncturing the battery case in severe crashes. Leaking coolant contributed to electrical fires in two Volt batteries that were sitting for a t least a week after government crash tests.

GM also is adding a sensor to monitor coolant levels and will attach a bracket to keep the coolant tank from leaking.

A Dec. 22 government crash test – after the changes were made to the car -- did not cause intrusion into the battery case and the coolant did not leak. Those are the two events that can combine to cause a fire. NHTSA monitored the Volt for weeks afterward and the car was fine.

No Volts on the road have caught fire.

GM sold 7,671 Volts in 2011, including 1,529 in December, which was also a strong month for fleet sales.

About 250 nervous Volt owners requested a loaner vehicle or took GM up on its offer to sell their Volt back to the company after learning about the potential fire hazard.