All seven victims have a "familial relationship" to the suspect, identified as 35-year-old Bradley William Stone, said prosecutors in Montgomery County, Pennsylvania yesterday.
Police said Stone -- possibly wearing military fatigues -- should be considered "armed and dangerous."
No possible motive was given for the shootings.
One of the dead is believed to be a woman who was killed in her house in the middle of the night. Her neighbour told local media the suspect then left the home with her children.
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The shootings took place at three different locations in suburban Philadelphia -- Souderton, Lansdale and Lower Salford.
Police were searching in a fourth location, the town of Pennsburg, where Stone is a resident, prosecutors said.
Reinforcements were rushed in to aid the manhunt for Stone, who uses a cane or a walker, according to police.