Japanese authorities failed to disclose US data about the spread of radiation spewing from a crippled nuclear plant last year, leaving some evacuees fleeing in the same direction as the radioactive emissions.
News that Japan's nuclear watchdog and the science and technology ministry sat on the information collected by US military aircraft - another sign of the chaos at the time - is likely to add to mistrust of nuclear power just days after the government approved the restart of two idled reactors.
A March 11, 2011, earthquake and tsunami devastated the Fukushima nuclear plant north of Tokyo, triggering explosions and meltdowns and causing about 160,000 people to flee the worst nuclear accident since Chernobyl in 1986.
US military aircraft gathered radiation data from March 17-19 over a 45km radius and found that people in an area about 25km northwest of the plant - where some people were moving - were exposed to the annual permissible level of radiation within eight hours, Japanese media said.
The information was passed to the Nuclear and Industrial Safety Agency (NISA) and the science and technology ministry by Japan's Foreign Ministry but neither agency passed it to the prime minister's office, which was overseeing the evacuations.
"It is extremely regrettable that this information was not shared or utilised properly within the government and I have no words to apologise, especially to the disaster victims," Industry Minister Yukio Edano, top government spokesman during the crisis, told a news conference.
The government had previously admitted that it failed to quickly disclose computer forecasts showing the direction radioactive material would disseminate, due to poor internal communication.
The result was that thousands fled in the same direction as the radioactive material was drifting.
Two panels of experts, one appointed by the executive branch and the other by parliament, are wrapping up investigations into the causes and responses to the Fukushima crisis.
Edano declined to comment on whether any government officials would be fired or otherwise punished after the panels' reports are issued.
Japan will also set up a new nuclear regulator in a few months after the expected passage of an enabling law, part of efforts to repair shattered trust in a regulatory regime long characterised by cosy ties between bureaucrats and utilities.
All of Japan's 50 reactors have gone off line for safety checks and maintenance since the disaster.
But despite public opposition, the government on Saturday approved the resumption of operations at two plants in western Japan to avert a power crunch.