Japan's nuclear regulator says it believes that the operator of the crippled Fukushima Daiichi nuclear plant failed to properly vent a vessel containing one of the plant's reactors in the days after the March 2011 accident.
Tokyo Electric Power Company attempted to vent the containment vessel of the No.2 reactor to lower interior pressure and prevent equipment from being damaged.
The Nuclear Regulation Authority said on Thursday that its analysts examined parts of piping used in the venting.
They said no significant contamination was found on the piping, showing that radioactive gas was not released through the pipes.
The regulator says it believes the venting was not conducted as planned due to unknown reasons.
An investigation by TEPCO also showed a similar result.
It is believed that a massive amount of radioactive gas was released from the No.2 reactor into the environment, but the source of the leak has not been determined.
The regulator plans to look into the level of radioactivity inside the No.3 reactor next month.