The gold coins, 1933 "double eagles" that were never circulated, could be worth millions of dollars apiece. A comparable one sold for $7.59 million in 2002 -- the highest price ever paid for a coin.
Plaintiffs Joan S. Langbord and her two sons say they discovered the cache in 2003 in a safety deposit box belonging to her late father, Philadelphia jeweler Israel Switt.
They approached the Mint the next year and agreed to turn them over to be authenticated, the Langbords say. But the Mint -- after vouching for them -- refuses to return them on grounds they were stolen U.S. property.
"Plaintiffs fail ... to plead any fact to support their implication that Switt legally obtained the 1933 Double Eagles," Assistant U.S. Attorney Joel M. Sweet wrote in the brief filed Friday. "(That) supports a reasonable inference that Switt obtained the 1933 Double Eagles knowing that they were stolen property."
Langbord, 76, still works at her father's store on Jeweler's Row, a few blocks from the Mint. She was out of the country Tuesday and could not be reached for comment, a business partner said.
In her December 2006 suit, she and sons Roy Langbord of New York City and David Langbord of Virginia Beach, Va., ask for the return of the coins or a settlement of up to $40 million.
At a minimum, they say, they deserve a trial over the alleged seizure.
Prosecutors argue there was no forfeiture involved because the Langbords never had rightful ownership in the first place.
"A thief cannot convey good title to stolen property," Sweet wrote.
More than 445,000 of the gold double eagles were minted in 1933, but they were melted into gold bars after President Franklin D. Roosevelt took the country off the gold standard.
In 1944, the Secret Service traced 10 separate double eagle coins that had surfaced to Switt. He acknowledged selling nine of the coins but said he did not recall how he had gotten them.
"Every single one of the 1933 Double Eagles traced back to Israel Switt," prosecutors said.
Switt was not prosecuted because the statute of limitations had run out. However, he was convicted of violating the Gold Reserve Act of 1934 in a separate case, and had his license to deal scrap gold -- which involved him with the Mint on occasion -- revoked.
Prosecutors also argue in their brief that someone other than Switt stashed the coins in the bank deposit box. The Wachovia Bank box was rented in 1996, six years after Switt died, they said.
And the family did not list the coins or pay taxes on them when his will was executed, they said.
Roy Langbord, an entertainment lawyer in New York, referred questions to the family's lawyer. His brother did not immediately return messages.
"We strongly disagree with the government's positions, and we're looking forward to making our arguments to the court," said family lawyer Eric Tirschwell.
Double eagles, first struck in 1850, are so named because they had a face value of $20, twice the amount of gold coins known as eagles.
Two of the 1933 coins, which feature a flying eagle on one side and a figure representing liberty on the other, were permitted to be spared from the melting pot and are at the Smithsonian Institution, but a handful of others also mysteriously survived.
The coins at the center of the lawsuit were briefly displayed last year for an American Numismatic Association's convention in Denver. They have been secured at the U.S. Bullion Depository in Fort Knox, Ky.