Ministry officials said public-safety investigators examining the fires believe lithium-ion batteries may have caused the iPod nanos to overheat. The ministry said Apple is aware of 14 additional cases in Japan of iPod batteries overheating.

"We are not in the position to speculate on the outcome of the investigation," Hiroyuki Yoshitsune, a Ministry official, told Agence France Press. "But after several incidents like these, it would be appropriate for Apple to take some measures to raise the public's awareness."

Lithium-ion batteries, found in many computers and mobile phones, have a higher power density than nickel-based batteries, according to Apple. This higher power provides a longer battery life. Apple warns on its Web site that iPod owners should keep the music players out of the sun or a hot car because heat will degrade the battery's performance.

Investigating the Cause

It is not clear whether the iPod nanos believed to have caused the fires were basking in the sun. Short-circuiting a lithium-ion battery can cause it to ignite or explode.

Officials have pointed to four specific models as the cause of the incidents -- MA004J/A, MA005J/A, MA009J/A and MA107J/A. More than 1.8 million of these players were sold in Japan between September 2005 and September 2006, according to published reports.

Quasi-government safety officials and Apple, according to published reports, will work together to find the specific cause of the overheating. Ministry officials are also asking Apple to improve its technology to avoid any additional fires.

Batteries a Cause for Concern

Lithium-Ion batteries have been the core problem behind many technological recalls in recent years.