The incident occurred in Tangi tehsil of Khyber Pakhtunkhwa when the accused Gulab Ahmed stormed his uncle's house and fired recklessly, killing 10 members of the family, local police said.
The reason behind the murders is stated to be a marriage dispute, police said.
The deceased include five women, two children and uncle of the accused besides two others.
The bodies have been shifted to Tangi Tehsil Hospital.
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The wife of the accused had moved to her parent's house in Tangi tehsil and upon learning about the murders of their son-in-law's father and brother, the parents refused to send her back to her husband.
The refusal provoked the culprit into killing his in-laws, police said, adding that the assassin had fled the scene.
Police have started a hunt to nab the accused killer.