Washington ( CNN ) -- The grounding of Boeing Dreamliners entered its second week with the company and investigators working non-stop in the United States and Japan to try to pinpoint fire risk in the 787 electrical system . U.S. aviation authorities are evaluating data but still do not have a handle yet on the cause of a battery fire on one Dreamliner and the reason behind a related incident on another plane that prompted regulators worldwide to idle the $ 200 million wide body until further notice . With battery and related components under microscopic examination and a new round of testing at manufacturing facilities underway this week , U.S. government officials at the center of the issue expressed eagerness to get to the bottom of the matter . But they also said it was crucial to let experts do a thorough job and the plane be re-certified as safe before lifting the order barring further flight . `` We do n't know what is causing these incidents yet , '' Federal Aviation Administration chief Michael Huerta told reporters at an industry event in Washington on Wednesday . `` We need to let them finish their work , '' Transportation Secretary Ray LaHood said at the same event . `` These are expert people . They 'll get to the bottom of it and then we 'll let all of you know what they find out , '' LaHood said . Experts : Quick fix for Dreamliner looks less likely . Boeing said in a statement that its technical experts are working `` around the clock '' and are focused on `` resolving the issue '' and returning the 787 to service . `` We are working this issue tirelessly in cooperation with our customers and the appropriate regulatory and investigative authorities . The company is eager to see both investigative groups continue their efforts and determine the cause of these events , and support their thorough resolution , '' said company spokesman Marc Birtel . A battery-related fire in the underbelly of an empty Japan Airlines 787 on the tarmac in Boston on January 7 triggered an FAA review of the aircraft . But regulators said the plane was still safe to fly . The FAA then grounded the fleet on January 16 after an emergency landing by an All Nippon Airways flight in Japan that was prompted by a battery alarm and a report of a burning smell . Analysis of a flight recorder from that plane showed that a lithium-ion battery experienced a sudden drop in voltage and was not overcharged , the Japan Transport Safety Board said Wednesday . Opinion : How Boeing can bounce back from Dreamliner problems . The agency is still investigating the cause of the battery 's sharp fall in voltage , Norihito Goto , its chairman , said at a news conference . GS Yuasa , the battery 's maker , is now disassembling the device to examine it in detail . The Japanese findings come after the U.S. National Transportation Safety Board said over the weekend that the battery in the Boston fire had not been overcharged , eliminating a relatively simple potential cause but adding new complexity to the investigation . Additional details released by the safety board said all eight cells of the lithium-ion battery involved in the Boston 787 fire showed varying degrees of thermal damage . Investigators scanned and dismantled six of them to expose electrodes for microscopic examination . Those batteries are used to power electrical systems , especially when the plane 's engines are idle . Dreamliner woes explained . The U.S. safety board also oversaw a series of new tests on battery systems and related components with manufacturers on Tuesday in Arizona . In Washington on Wednesday , U.S. officials were asked about the timing of the very unusual order to ground a fleet of planes . `` On the day we said the planes were safe , they were . On the day we announced that there was another incident and we were grounding the plane , we felt this was the time to ground the planes , '' LaHood said . Huerta explained that the second incident occurred during a flight . `` And that for us was an important consideration where we needed to identify what was causing these power- and battery-related incidents , '' he said . The FAA order applied directly to the six Dreamliners operated by United Airlines , the only ones flown by U.S. carriers . But its directive was picked up aviation authorities globally and requires that the planes be deemed safe before they fly again . Although there are only 50 wide body Dreamliners in service , there are several hundred on order . The world 's largest aircraft manufacturer has made a big bet on the most technologically advanced airliner . The 787 is highly touted because of its mostly lightweight carbon fiber construction , which airlines expect will help them save billions in fuel costs on long-haul routes . Its cockpit design and cabin appointments are state of the art . The high level of attention and widespread concern about its problems and image now are partly due to the Dreamliner 's difficult development . The 787 program at Boeing was plagued by cost overruns and production delays . The first one flew commercially last year . Why Dreamliner batteries worry experts . CNN 's Junko Ogura in Tokyo contributed to this report .
NEW : Japanese authorities say they experienced a sudden drop in voltage . U.S. government officials evaluating data as part of investigation of the problems . Aircraft were grounded last week after incidents in Tokyo , another incident in Japan . Fifty Dreamliners in service globally , but Boeing has orders for several hundred .