Recently disclosed FBI documents have revealed that an excavation almost three years ago was a search for lost Civil War-era gold.

The secret dig was carried-out in Pennsylvania three years ago this month, after a father-and-son duo, Dennis and Kem Parada, provided a tip-off to the federal agency.

Documents revealing the FBI dig were disclosed in court documents in recent days, following a lawsuit brought by the father-and-son against the FBI, who claimed nothing was found at the location.

The site, about 135 miles (220 kilometers) northeast of Pittsburgh, is supposedly the spot where a Civil War-era shipment of Union gold was either lost or stolen on its way to the US Mint in Philadelphia in 1863.

Until the lawsuit, the FBI had long refused to disclose why it went digging in the area, called Dent’s Run, and described the court-authorised excavation of “what evidence suggested may have been a cultural heritage site.”

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But the Parada family believed the federal agency was hiding something about the dig, which recently disclosed documents confirmed as a search for a “cache” that was “3x5x8 (feet) to 5x5x8” in size.

The email, which was sent to the FBI by K.T. Newton, an assistant US attorney in Philadelphia in 2018, was marked “Confidential”, the Associated Press reported.

The Paradas also allege that an FBI agent revealed to them, as well as a geophysical consultant sent to initially investigate the site, that there was a “mass”. Both parties are due to press their claims further in a press conference on Wednesday.

The “mass” was described to them as being located “one or two feet off Denny’s sweet spot,” in reference to the author of “Rebel Gold,” a book exploring the possibility of buried Civil War-era caches of gold and silver.

A spokesperson for the FBI said last week that “The FBI unequivocally rejects any claims or speculation to the contrary.”

Bill Cluck, the lawyer for the Prada family, is pressing further access to some 2,4000 pages of government documents in relation to the dig, which could reveal more information, through a Freedom of Information request.