Technicians at a nuclear plant that shut down in Japan's worst nuclear accident initially disregarded warning signs because they suspected a gauge measuring radioactivity was broken, officials said Monday.

A negligible amount of radiation escaped into the atmosphere Saturday before an emergency system shut off the Mihama nuclear plant in Fukui, western Japan, said Kenji Uchida, a spokesman for the plant's operator, Kansai Electric Power Co.The radiation poses no threat to humans or the environment, said Uchida. Earlier, officials had said no radiation escaped in the accident, which was blamed on cracks in steam generator equipment.

Katsuaki Shibata, deputy director of the government's Nuclear Safety Policy Research office, said the amount of radiation escaping from Fukui, though minimal, was the largest accidental release ever recorded in Japan.

Shibata said it exceeded that released from a 1981 incident at another nuclear power plant in western Japan. In that incident, contaminated water leaked from a pipe at a nuclear power plant in Tsuruga on the coast of the Sea of Japan after workers neglected to shut a valve.

Earlier news reports had said erroneously that previous accidental leaks had been larger.

Before Saturday's accident, Fukui had been shut down three times by malfunctioning emergency systems.

The plant had its annual mandatory inspection in July, and the Energy Agency said it may consider tightening inspections depending on the investigation of the latest incident.