The preliminary report by a team from the International Atomic Energy Agency also said the tsunami hazard was underestimated at several other nuclear facilities in Japan, and called for experts worldwide to learn from the disaster to avert future accidents.
The report said the earthquake and tsunami were the direct cause of the power outages and communications blackouts that ensued.
The report said the potential size of the tsunami that hit the plant, estimated at as high as 49 feet, was not sufficiently planned for and "overwhelmed" the plant's defenses. Further, the report suggested that long-term efforts to monitor the health and exposure levels of workers and the public "would be beneficial."
At the same time, the inspectors are praising Japan for the overall response to the crisis.
"Japan's response to the nuclear accident has been exemplary, particularly (as) illustrated by the dedicated, determined and expert staff working under exceptional conditions," the report said. It also praised the evacuation of those living near the plant as "impressive and well-organized."
Mike Weightman, the IAEA team leader, said the IAEA team focused on finding lessons from the crisis that can be applied around the world.
"You can make nuclear plants safe against natural events, but you have to understand those events," Weightman said.
Weightman said Japanese officials offered their full cooperation, and his team was provided access to the plant and officials and answers to their questions.
The Fukushima Dai-ichi facility was crippled when a huge tsunami generated by a magnitude 9.0 earthquake hit Japan on March 11. It suffered explosions, fires and meltdowns in the days after the tsunami. More than 80,000 people have been evacuated from its vicinity.
The Associated Press contributed to this story