The blast Thursday evening, just before the fireworks display was to start, may have been caused by a food stall vendor putting gasoline into an electricity generator without turning it off, said the deputy head of Kyoto's Fukuchiyama police station, who gave his surname as Okamoto. He said an investigation was underway.
Eighteen of those hurt suffered major burns and other serious injuries, including a 13-year-old boy and a 10-year-old boy, he said.
The annual festival in Fukuchiyama, Kyoto prefecture, was canceled after the explosion. Last year, it drew around 110,000 spectators.
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