TOKYO—The immediate danger of explosions or major radiation leaks at the Fukushima Daiichi nuclear-power plant has receded, a top Japanese administration official said, even as the government and the plant's operator battle to keep it stabilized.

Goshi Hosono, Prime Minister Naoto Kan's special adviser responsible for managing the accident, also said in a weekend interview with The Wall Street Journal that Japanese officials and lawmakers are starting to look into potential causes and the handling of the accident, which spread...