Published: Jan 23, 2013 11:08 PM EST

TOKYO, JAPAN- Japan's top safety official said on Wednesday that the damaged battery in the All Nippon Airways (ANA) 787 Dreamliner that made an emergency landing last week did not appear to be overcharged, although he did not rule out the possibility as the battery underwent further tests at the country's space agency.

Japan Transport Safety Board (JSTB) chairman Norihiro Goto said that while they had not seen any evidence of overcharging they still did not have all the data to make a complete and final judgment.

"We haven't seen direct overcharging. But in order to fully reach a conclusion as to whether there was or wasn't, its still somewhat premature," Goto told reporters at a news conference in Tokyo.

On Sunday (January 20), U.S. safety investigators ruled out excess voltage as the cause of a battery fire in a separate incident earlier this month involving the passenger jet operated by Japan Airlines (JAL) and said they were expanding the probe to look at the battery's charger and the auxiliary power unit.

Goto said he was surprised the airliner's problem was with the electrical system.

"The 787 uses a large amounts of composite materials, up to 70 percent, in areas such as the main wing to reduce weight. Japanese companies have also been very involved. I've thought that if something were to go wrong then it would be with one of the things that used a large amount of these new materials. I never imagined in my dreams that there'd be a problem with the electrical system," he said.

Boeing said on Friday (January 18) it would continue building the carbon-composite 787, but put deliveries on hold until the U.S. Federal
Aviation Administration approves and implements a plan to ensure the safety of potentially flammable lithium-ion batteries.

The U.S. National Transportation Safety Board (NTSB) said the Japanese agency was participating in its investigation of the Boston incident, while NTSB officials were helping the agency with its investigation of the emergency landing in Japan. Both investigations were ongoing with the Japanese agency enlisting the help of JAXA, the Japanese space agency.

"At first we had some trouble in figuring out exactly what type of investigative stance to take. After talking with JAXA however, we realized
that we had the same type of investigative equipment used in Boston, in other words a CT scanner, and at that point our options opened up," Goto said.

Japan is the biggest market so far for the 787, with ANA and JAL operating 24 of the 290-seat wide-bodied planes. Boeing has orders for almost 850 of the planes, which have the most complex electrical systems of any planes on the market.