SEATTLE/TOKYO -- In 2007, U.S. regulators cleared Boeings use of a highly flammable battery in the 787 Dreamliner, deciding it was safe to let the lithium-ion battery burn out if it caught fire mid-air as long as the flames were contained, and smoke and fumes vented properly, according to documents reviewed by Reuters.

Fire risk on planes has always been a major concern, especially given the amount of fuel they carry and the heat generated by jet engines. U.S. aviation standards require planes to have numerous on-board fire suppression systems.

But through a review of government documents and interviews with aviation and battery experts, Reuters found that the Federal Aviation Administration granted the Dreamliner special conditions and said its contain-and-vent system was sufficient to control the build-up of explosive or toxic gases, except in situations considered "extremely remote."

The FAAs 2007 decision is now coming under scrutiny after the lithium-ion batteries in two 787 planes failed within days of each other, sparking a fire in one case in Boston, and generating warnings and an acrid smell that prompted the pilots of the second plane to make an emergency landing in Japan.

A key U.S. Senate committee plans to hold a hearing in the coming weeks to examine aviation safety oversight and the FAAs certification of the 787, an aide to the Senate Commerce, Science and Transportation Committee told Reuters on Tuesday.