TOKYO - In the latest sign that Japan's nuclear nightmare at the stricken Fukushima Daiichi power plant is not over, officials Wednesday detected an ominous radioactive gas that suggested possible nuclear fission at one of the reactors.

Officials from the Tokyo Electric Power Co. acknowledged that they had detected signs of the gas xenon, which they said could be the byproduct of a nuclear reaction. They downplayed the discovery, insisting it had not led to a rise in the reactor's temperature, pressure or radiation levels. They added that the presence of xenon would not delay ongoing efforts to cool the reactor.

As a precaution, workers injected boric acid, a substance that neutralizes nuclear fission, through the facilities' cooling pipes, the utility said.

The plant was damaged during a massive tsunami triggered by a March 11 earthquake that struck the coastal region several hundred miles northeast of Tokyo.