Japan's nuclear regulator has maintained its view that multiple blasts occurred at the Fukushima Daiichi nuclear power plant's No.3 reactor following the earthquake and tsunami in 2011.
The plant suffered meltdowns from the accident. Three reactor buildings were severely damaged by hydrogen explosions.
On Thursday, the Nuclear Regulation Authority along with experts analyzed images taken at the No.3 reactor.
Officials say the state of buckled beams on the third floor indicates an instantaneous wind pressure of up to 5 atmospheres. The officials say such force can damage concrete structures and collapse wooden buildings.
They also studied the only footage of the No.3 reactor in the explosion, which was captured by a TV station in Fukushima Prefecture.
The officials say an analysis of image processing shows the first blast damaged the fourth floor. They say an ensuing fire on the uppermost fifth floor caused the remaining hydrogen to explode, which caused black smoke to emerge.
The regulator resumed its probe into the cause of the accident last year. It plans to draw up a report as early as next month.