The June 25 accidental firing by an Air Self-Defense Force F-4 fighter was caused by damage to electrical wiring for weapons control apparently inflicted when a drill hit the wiring during maintenance work, an ASDF team looking into the case said Tuesday.

ASDF officials said they discovered damage to the insulating cover of an electrical circuit that controls the aircraft's weapons system, adding that metal wire was exposed from the breakage.

During maintenance work, mechanics are supposed to fasten rivets with a drill right above where the bundle of wiring is located. A probe of the aircraft in question showed signs that a hole was mistakenly made near the spot, and the ASDF officials concluded that the damage was accidentally caused during previous maintenance on the jet.

The exposed wiring caused a short circuit, which resulted in the accidental firing of the F-4 EJ Kai fighter's 20mm cannon during a shooting exercise in Hokkaido, they said.

The ASDF officials, however, said they could not specify when the riveting was done because detailed records of maintenance work were not available. The aircraft last underwent an overhaul in 1994.

Ikuo Tootake, ASDF chief of staff, said, "We would like to explain in detail about this investigation result to local citizens to whom we caused trouble." The ASDF jet fired off 188 rounds into a civilian area during a drill at the Shimamatsu air-to-ground shooting range in western Hokkaido. No one was injured in the incident.