The panel presented on Friday a draft interim report on its probe into the accident.
The experts' conclusion differs from that of a Diet commission about the loss of an emergency power source at the Number One reactor at the Fukushima Daiichi nuclear power plant.
In its report released in July 2012, the commission suggested the possibility that the massive earthquake caused the reactor to lose power before the tsunami hit the plant.
But the expert panel says data found after the report was issued shows that the arrival of the tsunami coincided with the time the reactor lost power.
The panel rejects the conclusion by the Diet commission that the quake damaged pipes of the reactor and led to the meltdown.
The experts say they will continue their probe and interview former members of the commission before releasing a final report.