(Reuters) - The National Transportation Safety Board is examining the certification and testing of the lithium-ion battery system on the Boeing Co 787, but still has not found the root cause of a January fire, the board said on Thursday. The NTSB's 39-page "interim factual report" provides extensive detail on the testing performed on the battery that burned on a parked jet shortly after landing at Boston's Logan International Airpoprt. But it also made clear that investigators remain a long way from understanding why it happened in the first place. ...