In its statement, the agency said it was too early to tell whether the investigation would lead to a recall of vehicles or parts, but added that it would be notifying consumers if it identified a significant safety risk and making sure General Motors did the same.

The first fire happened three weeks after a Volt was side impact tested in May, with the cause attributed to a damaged battery pack and ruptured coolant line. Since then, the NHTSA has been working with the Department of Energy, Department of Defence and General Motors to recreate the conditions and help provide emergency services with post-accident protocols.

As part of its studies, it conducted side impact tests on two more Volts, then rotated them to simulate a rollover. The first car, tested on the 16th November, showed a temporary increase in battery temperatures a day after the test. On the second car, the battery pack started smoking and sparking after it had been rotated through 180 degrees. Forensic testing will continue this week.

‘NHTSA is not aware of any roadway crashes that have resulted in battery-related fires in Chevy Volts or other vehicles powered by lithium-ion batteries. However, the agency is concerned that damage to the Volt's batteries as part of three tests that are explicitly designed to replicate real-world crash scenarios have resulted in fire,’ the agency said in its statement.

‘NHTSA is therefore opening a safety defect investigation of Chevy Volts, which could experience a battery-related fire following a crash. Chevy Volt owners whose vehicles have not been in a serious crash do not have reason for concern.’