Police in Granite Shoals say the suspect was detained following the fire Wednesday. The youth, whose name hasn't been released, remains in juvenile custody on a charge of arson causing bodily injury or death.
Granite Shoals police didn't immediately release the names of the child who died or the injured woman, who's been transported to a San Antonio hospital for treatment of burns. Investigators also did not immediately confirm whether the boys and the woman are related.
Authorities have not released further information on how the fire apparently started.
Granite Shoals is 50 miles northwest of Austin.
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