Hitachi police spokesperson Masataka Suzuki said the six victims found after the fire on Friday are believed to be the wife and five children of the man, Hirobumi Komatsu.
Police said a woman and four boys were found dead in the building in Hitachi, a town northeast of Tokyo. A girl found with several stab wounds was rushed to a hospital, where she was declared dead.
Suzuki said Komatsu has told police he set the fire. The case remains under investigation.
Japanese media say the children were ages 3 through 11.
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