Waves crash into Japan's Fukushima nuclear power plant during the tsunami in March last year, triggering the worst atomic disaster since Chernobyl.
But a new report said on Thursday that the crisis developed into a "man-made" preventable disaster.
An independent commission appointed by Japan's parliament said in their final report that the catastrophe was the result of "collusion" between the government, regulators and the plant operator.
The report underscored the cosy ties among these groups that critics have blamed for turning a natural disaster into a man-made crisis.
The commission pointed to numerous missed opportunities to take steps to prevent the disaster.
Operator Tepco came under heavy criticism in the report, including for putting cost-cutting steps ahead of safety measures.
The panel also pointed to problems in the response of then prime minister Naoto Kan.
The report - which involved over 900 hours of interviews and over a thousand people - is the first of three probes looking into the disaster.
Simon Hanna, Reuters.
