According to the Eastern Pennsylvania Preservation Society, the remains belong to the Hoods.
It is believed that the historic mansion was part of the Underground Railroad.
Built in 1834, the mansion served as the family's summer home until 2017. John McClellan Hood built the mansion for his wife and 13 children.
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By the early 1900s, 15 family members were interred on the estate's grounds. But due to vandalism and grave robbing, the Eastern Pennsylvania Preservation Society says most of the remains were relocated to Philadelphia's Laurel Hill Cemetery.
Only the remains of John, his wife Elizabeth, his son, Washington, and John's brother Montgomery Hood remained interred at the estate.
The remains found recently have not been identified.
The property is owned by a Brooklyn, New York, developer who has proposed a plan to demolish the home and build warehouses on the property.
"That plan would destroy the mansion, leave the mausoleum exposed and vulnerable, and erase the Hood family history," the Eastern Pennsylvania Preservation Society says.
The mansion went viral after the society offered it for free to anyone who could move it to a different location.
The Eastern Pennsylvania Preservation Society still hopes to save the historic land.
"Our goal is to keep the lines of communication open with the developer for the eventual subdivision of six acres of land that will allow the preservation of the Hood mansion, its history, and the historic ancestral burial ground of the Hoods," the society says.