Fires on two Boeing 787 Dreamliners were likely caused by overcharged lithium ion batteries, aviation safety and battery experts said Friday.

An investigator in Japan, where a 787 made an emergency landing earlier this week, said the charred insides of the plane's lithium ion battery show the battery received voltage in excess of its design limits.

The similarity of the burned battery from the All Nippon Airways flight to the burned battery in a Japan Airlines 787 that caught fire Jan. 7 while the jet was parked at Boston's Logan International Airport suggests a common cause, Japan transport ministry investigator Hideyo Kosugi said.

"If we compare data from the latest case here and that in the U.S., we can pretty much figure out what happened," Kosugi said.

American and foreign aviation authorities have grounded the 50 Dreamliners in service while the investigation continues.

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U.S. Transportation Secretary Ray LaHood said the 787 would not fly again until regulators were "1,000 percent sure" it was safe.

LaHood, who told a news conference on Jan. 11 he would not hesitate to fly on the plane himself, was asked by reporters Friday whether he regretted his prior statements.

"Last week it was safe," he replied. What has changed since then, he said, is the fact that another incident occurred involving the batteries.