(CNN) About 145 coffins believed to be part of a mid-20th century cemetery for the indigent were found buried under a Tampa, Florida, high school, officials said.

The Hillsborough County school district said Wednesday that geophysical technicians found "clear evidence" of burials at the Clarence Leon King High School campus.

School officials believe the caskets are part of Ridgewood Cemetery, a historic paupers' burial ground from the early 1940s that was owned by the city, according to the Florida Genealogical Society . King High School opened in 1960, according to the school's website.

Tanya Arja, a spokeswoman with the school district, said officials began investigating last month after a person told a school board member of the possible location of the cemetery.

For nearly two weeks, technicians hired by the district used ground penetrating radar to confirm the location of the caskets. The coffins are buried 3-5 feet deep in about an acre of open land, and near a small building used by school's agricultural program, the district said.

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