The final report from the Japanese government investigating the causes of the Fukushima nuclear disaster is in. It's in English at the first link above, so read the entire thing if you want to get into the nitty gritty details, but here's the final conclusion:
The disaster was "manmade" the report says. The Tokyo Electric Power Company "effectively betrayed the nation's right to be safe from nuclear accidents." No specific individual was responsible for the disaster, rather "the root causes were the organizational and regulatory systems that supported faulty rationales for decisions and actions."
The Fukushima Daiichi nuclear power plant was "incapable of withstanding the earthquake and tsunami," with the plant's operator, the Japanese nuclear regulatory bodies, and government body promoting nuclear power "all failed to develop the most basic safety requirements" for the plant.
Since 2006, the report goes on to say, regulators and TEPCO both knew that a tsunami might well cause a total outage of electricity at the plant, that if the seawater pumps went down in a tsunami reactor core damage was possible.
As far as whether the tsunami or the earthquake itself that stuck the fatal blow, as it were, the report says TEPCO was too quick to blame the tsunami and that further investigation is needed to determine what damage was specific to the earthquake.
Finally, the report has some very harsh words, with application to far more areas of governance and parallels with the banking industry, other energy disasters, other industrial disasters, as well as basic corporatism:
The underlying issue is the social structure that results in "regulatory capture," and the organizational, institutional, and legal framework that allows individuals to justify their own actions, hide them when inconvenient, and leave no records to avoid responsibility. Across the board, the Commission found ignorance and arrogance unforgivable for anyone or any organization that deals with nuclear power. We found a disregard for global trends and a disregard for public safety. We found a habit of adherence to conditions based on conventional procedures and prior practices, with a priority on avoiding risk to the organization. We found an organization-driven mind-set that prioritized benefits to the organization at the expense of the public.