TOKYO (AFP) ― Japanese and American safety inspectors were Monday probing the company that makes batteries for Boeing’s Dreamliner after the aircraft’s worldwide fleet was grounded over safety fears.

Aviation regulators were focusing on the lithium-ion batteries as the cause of a glitch that forced an All Nippon Airways flight into an emergency landing last week.

Despite the investigation shares in GS Yuasa, which manufactures the batteries for the Dreamliner’s advanced electronics systems, were up 0.95 percent to 318 yen in afternoon Tokyo trade.

The Japanese firm is just one of many contractors in a complex global chain that led to three years of delays before Boeing delivered its first 787 to ANA in 2011.

“Engineers from the FAA (U.S. Federal Aviation Administration), Boeing and our aviation bureau started a probe this morning that is mainly focusing on GS Yuasa’s production line,” said Yasuo Ishii, a transport ministry safety official.

“They are checking on whether there have been any issues in the production process. We still don’t know what caused the battery problem and so we are looking into all possibilities.”

Ishii said the inspection does not mean authorities think GS Yuasa, headquartered in the western city of Kyoto, was to blame for the problems.

Investigators on Friday released a picture showing the blackened remains of the battery in the ANA plane. They are also inspecting the aircraft’s black box which contains data from the flight that may help to assess how the battery was affected.

Boeing’s cutting-edge new planes suffered a series of glitches earlier this month, prompting a global alert from the FAA that led to the worldwide grounding of all 50 operational 787s.

The risk of fire from overheating powerpacks emerged as a major concern after pilots were forced to land a domestic ANA flight on Jan. 16 due to smoke apparently linked to the lithium-ion battery.

U.S. investigators probing a fire on a Japan Airlines 787 after it landed in Boston on Jan. 7 ruled out an overcharged battery as the cause, although the powerpacks were undergoing further tests.

On Monday, ANA said the Dreamliner’s grounding forced it to cancel 335 flights up to next Sunday, affecting nearly 48,000 passengers.

Cancellations on domestic routes from Jan. 16 to Jan. 27 amount to 292 flights for 44,074 passengers, while 43 international flights have been cut, affecting 3,778 passengers, the airline said.

Flights affected include those from Tokyo to San Jose, Seattle and Beijing.

ANA also said it would not receive a Dreamliner that was on order due to the grounding, but declined to say if it would cancel any purchases it has made to date.

“We were supposed to receive our 18th plane from Boeing later this month,” a company spokesman said.

“But as the suspension continues, it may be difficult for us to receive it as scheduled. So far, we have no plans to review anything related to our 787 contracts,” he added.

ANA and its domestic rival JAL are key Dreamliner customers with Japan’s two biggest airlines having ordered a combined 111 aircraft so far.