The blaze started early Sunday morning on Georgia Street in Vallejo and was immediately considered suspicious.
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Police say physical and electronic evidence as well as witness and victim statements led them to their conclusion.
RELATED: 2 children, woman killed in 'suspicious' house fire in Vallejo
Mau Dao's death is being classified by police as a suicide that led to two homicides.
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"This was a tragic event for all involved. Incidents of this nature are exceedingly difficult for the family, but also for the community and its first responders, " wrote Vallejo Police on Thursday.
RELATED: Beloved Vallejo twins among 3 killed in 'suspicious' house fire
Dao's other 11-year-old daughter was able to escape the blaze. She was hurt but is expected to make a full recovery. Police say she is now staying with her father.
"It is horrible how they died. This is not what they deserved," said Angelo Galvez, a friend of 14-year-old twins Trinh and Tram Tran.
A GoFundMe has been set up for the family.