The National Highway Traffic Safety Administration has been checking the car including crash tests.
Authorities said the tests resulted in fires, although there haven't been any real-world instances of battery-related fires after accidents.
General Motors executives said they have been working with the federal agency as it completes its investigation.
GM, which was informed of the investigation on Friday, said in a statement that the Volt "is safe and does not present undue risk as part of normal operation or immediately after a severe crash."
The latest fires are in addition to a battery fire in a crash-tested Volt six months ago.
Lithium-ion batteries, which are rechargeable, have been the subject of several recalls of consumer electronics. Millions of laptop batteries made by Sony Corp. for Apple Inc., Dell Inc., Lenovo Group Ltd. and other PC makers were recalled in 2006 and 2007 after it was discovered that they could overheat and ignite.
The Associated Press contributed to this report.