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The crisis at the Fukushima nuclear plant was "a profoundly man-made disaster", a Japanese parliamentary panel has said in a report.
The disaster "could and should have been foreseen and prevented" and its effects "mitigated by a more effective human response", it said.
The report catalogued serious deficiencies in both the government and plant operator Tepco's response.
It also blamed cultural conventions and a reluctance to question authority.
The six-reactor Fukushima Daiichi nuclear plant was badly damaged after the 11 March 2011 earthquake and tsunami knocked out cooling systems to reactors, leading to meltdowns and the release of radioactivity.
Tens of thousands of residents were evacuated from an exclusion zone around the plant as workers battled to bring reactors under control. Tepco declared the reactors stable in December 2011.
Members of the Fukushima Nuclear Accident Independent Investigation Commission were appointed to examine the handling of the crisis and make recommendations.
The investigation included 900 hours of hearings and interviews with more than 1,000 people.
In the panel's final report, its chairman said a multitude of errors and wilful negligence had left the plant unprepared for the earthquake and tsunami.
"Although triggered by these cataclysmic events, the subsequent accident at the Fukushima Daiichi Nuclear Power Plant cannot be regarded as a natural disaster," it said.
"It was a profoundly man-made disaster - that could and should have been foreseen and prevented."
After six months of investigation, the panel concluded that the disaster "was the result of collusion between the government, the regulators and Tepco" founded in the failure of regulatory systems.
It said that the situation at the plant worsened in the aftermath of the earthquake because government agencies "did not function correctly", with key roles left ambiguous.
It also highlighted communication failures between Tepco and the office of then Prime Minister Naoto Kan, whose visit to the site in the immediate aftermath of the earthquake "diverted" staff.