The Japan Atomic Energy Agency says 75 percent of the radioactive substances released from the Fukushima Daiichi Nuclear Power Plant came more than 4 days after the accident.

The government’s investigation has not released what happened during this period. Experts say the reason needs to be determined as to why massive amounts of radioactive materials continued to be released for a prolonged period.

The nuclear accident in Fukushima has been evaluated as the worst, at level 7, on a par with the Chernobyl accident in 1986, due to the large amount of radioactive substances that were released. But the details on how the substances were released remain unknown.

A research group at the Japan Atomic Energy Agency collected new data on radiation detected near the plant over time to analyze how radioactive materials were released into the air.

The research has found that an estimated 470,000 terabecquerels of radioactive substances had been released by the end of March 2011, when the discharge is believed to have mostly subsided.

The research group says 25 percent of the radioactive materials were released during the first 4 days of the accident, as the meltdown and hydrogen explosions were happening, while 75 percent were released over the 2-week period that followed.

The group also analyzed how the radioactive materials spread, using the climate data at the time. They found that contamination in places where former residents are still not allowed to return became serious on March 15th — 4 days after the accident.

They also say radioactive substances released between March 20th and 21st spread to a wider area, including the Kanto region, and are believed to have contaminated drinking water supplies.

The outcome of the analyses indicates that radioactive materials continued to be released after the first 4 days, which is believed to be the critical time during which the situation was deteriorating out of control.

The government’s investigation has focused on the first 4 days, and has not determined the cause of the massive release of radioactive substances following that period.

Masamichi Chino of the research group says the cause needs to be determined to prevent future accidents and to bring the situation under control quickly if another accident happens.

More than 120,000 people are still forced to live in temporary shelters.

Six municipalities remain off limits due to high levels of contamination.