It happened around 4:40 p.m. Saturday on the 300 block of East Ashmead Street in Philadelphia's Germantown neighborhood.
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Police say a 17-year-old male and his 9-year-old half-brother were playing video games when an argument ensued.
According to investigators, the teen was bullying and harassing the child and threatened him with a BB gun.
That's when police say the 9-year-old somehow found a handgun and shot his half-brother.
The teenager was rushed to an area hospital where he later died.
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It's still unclear how the child gained access to the gun.
There was no immediate word if charges would be filed.