A high concentration of liquid oxygen, used as rocket engine coolant, has been determined as being behind a fire that broke out earlier this month at a launchpad in southwestern Japan, an official close to the matter has said.

Mitsubishi Heavy Industries Ltd. had planned to launch the Japan Aerospace Exploration Agency's supply vessel Kounotori8, using an H-2B rocket, from the Tanegashima Space Center in Kagoshima Prefecture on Sept. 11. But it was canceled when a fire broke out about three hours before the launch at the foot of the pad.

Although liquid oxygen is normally dispersed by the wind, its concentration rose due to light winds at the time, subsequently making nearby objects highly flammable, the official, who spoke on condition of anonymity, said Friday.

It is possible the fire was sparked by static electricity or other reasons after the liquid oxygen became highly concentrated, the official added.

Burn marks were found on thermal insulation material used to protect a vent on the launchpad, which is designed to allow engine exhaust and flames to escape as the rocket takes off.

Although liquid oxygen is used as an oxidizing agent to burn hydrogen, which acts as fuel for the rocket, it is also used as a coolant for the engine after the tank is filled, according to the official.

Mitsubishi Heavy, which has assured it will take measures to prevent a recurrence, said it plans to launch the rocket around 1:30 a.m. Tuesday.

Kounotori8, named using the Japanese word for "white stork," was supposed to deliver about 5.3 tons of supplies to astronauts at the International Space Station, including food and water as well as batteries and devices needed for experiments, according to the space agency.