TOKYO -- The head of Japan's leading aviation workers union said it is unclear whether faulty circuit boards were responsible for a series of battery fires that led All Nippon Airways to ground its Boeing 787 aircraft.

“The circuit board case on April 7 was serious and caused damage to the surrounding area,” Japan Federation of Aviation Workers’ Unions secretary general Shozo Tsue wrote in a letter to Japan transport minister, adding that he doesn't know of a link between the PCBs and the fires.

“We can’t say if there was any connection between the circuit boards and the battery, but there have been lots of problems with the electrical system,” Kazuo Harigai, assistant secretary of the Japanese aviation workers’ union, added.

All Nippon is currently the leading customer of the Dreamliner jets.

The Boeing planes were grounded following damage to three circuit boards after lithium-ion batteries caught fire. Boeing hasn't definitively stated whether a cause has been determined. Last month, however, Boeing vice president and 787 chief project engineer Mike Sinnett said the fault haf been "traced back to a single lot of (circuit) boards manufactured at one time by a subtier supplier.”