Relatives went to the scene in the 4000 block of Knoxville near Scott, calling what occurred a family tragedy.
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When police arrived to the home, multiple family members ran outside, saying that the uncle had been assaulted.
But before officers could go inside, they realized the house was on fire.
Houston firefighters were able to rescue the uncle, who police said had been hit with a hammer.
He was taken to the hospital and is said to be in extremely critical condition.
The nephew had smoke inhalation, but was detained at the scene.
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According to authorities, the nephew hit his uncle with the hammer at least one time in the head before setting the fire.
The family told ABC13 that the suspect, believed to be 30 years old, suffers from mental illness. The uncle is in his late 40s.
Five people were in the home at the time of the incident, but it wasn't immediately known if that included any children.
Arson is investigating.
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