UNDATED (CNN) -- The grounding of the Boeing 787 Dreamliner is now entering its fourth week. And the National Transportation Safety Board now says a battery in the belly of the plane is the focus of the investigation into fires on board.
All 50 Dreamliners in the world have been essentially grounded for three weeks and scrutinized after a string of incidents.
The National Transportation Safety Board's investigation zeros in on the planes' lithium ion batteries.
One caught fire in a Japan Airlines plane on the tarmac in Boston. And another burned in an All Nippon Airways flying over Japan, forcing an emergency landing.
Investigators found the batteries on board both flights overheated and the battery in Boston showed signs of a short. NTSB chairwoman Deborah Hersman says, "Our investigators are examining, not just the battery failure, we're looking at what caused that battery failure but we're also looking at the certification process."
The Dreamliner is the first commercial plane to depend so heavily on lithium ion batteries. They hold more energy in a lighter, smaller space and were only allowed under an FAA condition that special safeguards were in place.
By reviewing the certification process, the NTSB hopes to learn how these batteries were determined to be safe for flight. Ultimately, it will be the FAA's decision when to lift the ban and allow Dreamliners to fly again.