The school, just over six miles (10 kilometers) from the plant, was not immediately cleared out. Quite the opposite. It was turned into a temporary evacuation center.
Reports from the forecast system were sent to Japan's nuclear safety agency, but the flow of data stopped there. Prime Minister Naoto Kan and others involved in declaring evacuation areas never saw the reports, and neither did local authorities. So thousands of people stayed for days in areas that the system had identified as high-risk, an Associated Press investigation has found.
At Karino Elementary in the town of Namie, about 400 students, teachers, parents and others gathered in the playground at the height of the nuclear crisis stemming from the March 11 earthquake and tsunami. Many ate rice balls and cooked in the open air.
They were never informed of the predictions that they were at risk. In an interview with the AP, Namie's mayor said it took more than 24 hours for him to realize - from watching TV - that the evacuees were in danger. He sent buses to move some of them out. But, unaware of the risks, they were taken to another part of town also forecast to be in the plume's path. Most were left to fend for themselves.
"When I think about it now, I am outraged," Principal Hidenori Arakawa said. "Our lives were put at risk."
Documents obtained by the AP, interviews with key officials and a review of other newly released documents and parliamentary transcripts indicate that the government's use of the forecast data was hamstrung by communication breakdowns and a lack of even a basic understanding of the system at the highest levels.
It's unclear how much radiation people might have been exposed to by staying in areas in the path of the radioactive plume, let alone whether any might suffer health problems from the exposure. It could be difficult to ever prove a connection: Health officials say they have no plans to prioritize radiation tests of those who were at the school.
But the breakdown may hold lessons for other countries with nuclear power plants because similar warning systems are used around the world. This was their first test in a major crisis.
The Japanese network - built in 1986 at a cost of $140 million (11 billion yen) - is known as SPEEDI, short for the System for Prediction of Environment Emergency Dose Information. It has radiation monitoring posts nationwide and has been tested in a number of drills, including one the prime minister led for the Hamaoka nuclear facility just last year.
Even so, according to the prime minister's office, Kan and his top advisers never asked for or received the data. Despite taking part in the Hamaoka drill, Kan admitted he didn't understand how SPEEDI worked or how valuable the data was.
"I had no idea what sort of information was available," he told Parliament on June 17. "I didn't know anything about it then, and there was no way I could make a judgment."
In two post-crisis assessments, a report to the International Atomic Energy Agency and an annual white paper on science and technology, his government has said the network "failed to perform its intended function."
A senior member of Kan's crisis team, Nuclear Safety Commission chief Haruki Madarame, went so far as to say the SPEEDI data was no better than "a mere weather report."
He said the predictions were of no value because they lacked accurate radiation readings. Some of the system's monitoring capabilities were compromised by the tsunami and ensuing power outages, and the utility that runs the Fukushima plant, TEPCO, did not provide readings of its own.
But SPEEDI officials say Madarame's position reflects a fundamental misunderstanding of what their system is designed to do.
When the amount of radioactivity that has been leaked is known, that is entered into its system, along with weather and terrain data, and a hazard map is generated. If the amount is not known - as was the case with Fukushima - a standard and relatively low value of one becquerel is used.
While that won't show the actual radiation risk, it will show the general pattern and direction of the plume. Then when the size of the leak becomes known, the map can be updated. If the actual leak turns out to be 100 becquerels, for example, the results would be multiplied by 100.
That technique allowed SPEEDI to produce reports hours before officials began venting disabled reactors - when there would have been less radiation to measure outside the nuclear plant even if the system's monitoring equipment had been working perfectly.
In the Fukushima case, later data proved the forecasts to be highly accurate. Most of Namie, for example, has since been declared too dangerous for habitation.
"We are offended by allegations that SPEEDI failed to function the way it was supposed to," Akira Tsubosaka, a senior official in charge of operations, told the AP. "SPEEDI was not used to determine evacuation zones. It should have been."
SPEEDI, run by the education and science ministry, provides its data to other government agencies such as the nuclear safety agency for passage up the chain and then dissemination to local authorities.
Officials won't say why that didn't happen, sticking to their position that the data was useless anyway.
But the government response has been sharply criticized by one of Kan's top science advisers, who later quit in protest, according to a confidential report to the prime minister that was obtained by the AP.
"The SPEEDI radiation forecasts were not properly utilized and a situation was invited in which residents were made vulnerable to more exposure than necessary," Toshiso Kosako, also a professor at the University of Tokyo, wrote in late April.
Ironically, low-level officials were quick to seek the SPEEDI data.
Bureaucrats familiar with SPEEDI commissioned at least 18 tailor-made forecasts in the first 24 hours, as the government was pushing TEPCO to open vents to avert an explosion.
The venting would release radioactive substances into the air. So, according to documents obtained by the AP, the forecasts included several to gauge that danger.
One issued at 3:53 a.m. - about 13 hours after the crisis began - predicted the plume would drift across Namie and several other towns.
The forecasts were relayed to the Nuclear and Industrial Safety Agency but they did not reach decision-makers.
In Japan, the legal responsibility for setting evacuation zones falls on the central government and the prime minister. Local officials then are tasked with implementing the orders.
Instead of following the patterns of radioactive dispersion suggested by SPEEDI, the central government simply set up a six-mile (10-kilometer) evacuation zone around the plant. That did not include a broad swath of land that SPEEDI predicted would be affected.
The mayors of two towns that have since been almost completely evacuated told the AP that the government did not inform them of even that decision - let alone provide SPEEDI data - so they had to act on their own. They said they were unable to assess the risks adequately because they were not privy to the SPEEDI reports.
"We got nothing until more than a week later," said Katsunobu Sakurai, the mayor of Minami-Soma. "People were unnecessarily exposed to possible dangers. We believe the central government must come clean on this."
"The first I heard of the 10-kilometer zone was when I saw the news on TV," said Namie Mayor Tamotsu Baba. Namie's municipal government has since been evacuated to Nihonmatsu, a city 30 miles (50 kilometers) from the plant.
Because Karino Elementary sits just outside the six-mile (10-kilometer) evacuation zone, it was used as a gathering area for evacuees.
Later in the day, at the mayor's order, some evacuees were taken by bus to another part of Namie called Tsushima, which SPEEDI data suggested was also dangerous. Others, including Principal Arakawa and his family, went in the same direction by car.
Masako Mori, a senior opposition member of Parliament from Fukushima, told the AP that two alternative routes would have led away from the areas identified as high-risk by SPEEDI. The third - to Tsushima - led along the plume's expected path.
"We didn't have any information. But it turns out we were taking the most dangerous route," Arakawa said. "None of us knew."
Mori said SPEEDI data should have been used to get people out of the area much faster.
The evacuees at shelters in the Tsushima district - including about 8,000 residents of Namie - were not told to move farther away until March 16, five days into the crisis.
Mori, who also is a trial lawyer, raised the possibility of lawsuits against the government.
"The government unnecessarily exposed people to radiation, failing to observe its legal obligation to protect the citizens," she said. "It could be held responsible for compensation for the possible damage caused by its errors."
Exposure to radiation can lead to a variety of cancers - as it did in Chernobyl. Babies, children and pregnant women are at the highest risk.
Mori, along with Namie's mayor and the school principal, are seeking full-body radiation tests for all children who were at the school. The tests measure internal exposure such as inhaled radioactive particles and could be key to understanding the health impact. But Fukushima health officials say they have no particular plan to test the Karino evacuees, because they don't have the resources and are instead focusing on groups, such as pregnant women, from the general area.
Thousands across the region have requested full-body tests, and only 340 have gotten them so far. None had dangerously high levels of contamination, though that does not rule out future health problems.
The Fukushima health office said it is looking into ways to speed up the process.