Bradley Stone, a 35-year-old Iraq War veteran, was suspected of killing his ex-wife and five of her relatives.
He had been sought in connection with the fatal shootings of six people near Philadelphia on December 15.
His ex-wife, her mother, her grandmother, her sister, her sister’s husband and their daughter were killed.
Montgomery County District Attorney Risa Vetri Ferman told reporters that police found Bradley Stone’s body about half a mile from his home in Pennsburg.
He died of “self-inflicted cutting wounds” to the centre of his body, she said.
Reports said the shootings may have been triggered by a custody dispute.
“[Ex-wife Nicole Stone] would tell anybody who would listen that he was going to kill her and that she was really afraid for her life,” Evan Weron, a neighbour in Harleysville, told the Associated Press news agency.
Bradley Stone had reportedly filed an emergency motion to the court earlier this month, although those records are currently sealed.
The shootings in Souderton, Lansdale and Harleysville began at about 04:00 local time on December 15.
The first took place at the home in Souderton of Bradley Stone’s former sister-in-law, Patricia Flick. She was killed along with her husband, Aaron Flick, and the couple’s daughter, Nina, 14. The couple’s 17-year-old son, Anthony Flick, was badly wounded.
He is currently in very serious but stable condition in hospital, authorities said.
The next shooting was at a house in nearby Lansdale where Nicole Stone’s mother, Joanne Hill, and grandmother Patricia Hill were killed.
Nicole Stone was later killed at her home in Harleysville. Neighbors reported seeing Bradley Stone fleeing with their two children and raised the alarm.
The children were found safe a short distance away, and Bradley Stone’s current wife and infant child were also secured.
It was the second large-scale manhunt in Pennsylvania in the last few months.
Eric Frein spent 48 days on the run after a September ambush in which he killed a police officer.
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