DETROIT - U.S. safety regulators are combing over five Chevrolet Volts to determine whether welded parts near the 400-pound battery contributed to fires that occurred following government crash tests.

The National Highway Traffic Safety Administration, in a memo on its website, said it wants to know if crashes caused changes in welded areas on the underside of the vehicle, where the battery rests, reported The Wall Street Journal.

NHTSA asked its test center to take pictures of the vehicle's "floorplan/crossmember near the battery tunnel," and asked that the work be done "at the "earliest opportunity."

General Motors Co. is working with NHTSA to address the issue that caused sparks or fires in three battery packs following crash tests by the agency.

GM engineers believe they can fix the battery and retrofit cars already on the road without an extensive redesign, people familiar with the situation said.

The company says a damaged coolant line is behind the problem.

In the crash tests, a break in the coolant line caused coolant to leak onto wiring in the battery. After time, the coolant crystallized, causing a short.

GM has said repeatedly that the cars are safe. NHTSA said Volt owners shouldn't worry. No incidents have been reported in real-world driving and the NHTSA fires happened after weeks or days.

Whether GM's fix is as straightforward as the company believes it to be will depend of NHTSA's determination of the risk involved in the cars.

If NHTSA determines the Volt presents "a serious risk of injury" it would trigger a process that would lead to a recall, according to the agency's guidelines.

If not, GM gets to decide what course of action it will take and would not typically require approval from NHTSA.