NIHONMATSU, Japan — Nobody knows whether Hiroshi Yokoyama’s elderly parents tried to outrun the tsunami that engulfed their home in Namie on the Fukushima coast a year ago.

But Mr. Yokoyama does know that he would have looked for them high and low, if not for a second disaster that unfolded at the nuclear power plant just a few miles away, forcing him to abandon his search.

As grieving families across the nation gathered Sunday to mark the anniversary of Japan’s 3/11 disasters — an earthquake and tsunami that ravaged the northeastern coast, killed almost 20,000 people and caused a huge nuclear radiation leak — some communities are still coming to terms with the calamity’s scale, complexity and lasting effects, and painful new revelations have shed light on how some of the victims died.

Last week, the police in the Futaba-gun region of Fukushima, which includes the damaged Fukushima Daiichi nuclear power station and the town of Namie, confirmed that a handful of tsunami survivors who were trapped in the rubble probably starved to death as rescuers fled the scene for fear of radiation. A month passed before rescuers were able to venture back into the exclusion zone set up in a 12-mile radius around the nuclear plant; the bodies of Mr. Yokoyama’s parents were not discovered until the summer....