TOKYO — In the darkest moments of last year’s nuclear accident, Japanese leaders did not know the actual extent of damage at the plant and secretly considered the possibility of evacuating Tokyo, even as they tried to play down the risks in public, an independent investigation into the accident disclosed on Monday.

The investigation by the Rebuild Japan Initiative Foundation, a new private policy organization, offers one of the most vivid accounts yet of how Japan teetered on the edge of an even larger nuclear crisis than the one that engulfed the Fukushima Daiichi Nuclear Power Plant. A team of 30 university professors, lawyers and journalists spent more than six months on the inquiry into Japan’s response to the triple meltdown at the plant, which followed a powerful earthquake and tsunami on March 11 that shut down the plant’s cooling systems.