"It sends a strong message" that the government will pursue thefts, "no matter how many years pass," Assistant U.S. Attorney Jacqueline Romero said after the verdict.
The coins are now in Fort Knox, Ky.
"Israel Switt and some of his friends stole 1933 double eagles from the Philadelphia Mint," Romero said during closing arguments Tuesday.
Barry Berke, the attorney for Switt's daughter, Joan Langboard and her two sons, Roy and David, declined to comment after the verdict. Appeals are expected. Roy Langbord shook a reporter's hand, but also declined comment. His mother was not in the courtroom.
Joan Langbord, 81, said on Tuesday that she had "no idea" how her father had obtained the 1933 double-eagle coins. A 1933 double eagle was sold in 2004 for $7.5 million, a record price for any coin.
The family contended they were legally entitled to the coins because there was no way to know exactly how the got out of the mint, and ended up in the Switt family's safety deposit box.
But Romero said the circumstantial evidence pointed to theft.
The Mint never legally issued the 1933 double eagles, and all 445,471 were accounted for in mint records, Romero said. In 1937, all the coins were supposed to have been melted down into bullion. But since then, at least 21 have surfaced, and all can be traced back to Switt, who Romero said had gotten them from a corrupt Mint official. Switt died in 1990.
Berke argued that the coins could have been legally obtained from the Mint during a "window of opportunity" in March and April 1933, and that government had not proved otherwise.
Langbord said she had discovered the coins in 2003 during one of her periodic visits to a safe-deposit box containing jewelry inherited from her mother.
"We did not think that was a credible story," Romero said after the verdict. Romero worked on the case for five yers, and prosecuted it in court with Assistant U.S. Attorney Nancy Rue.
U.S. District Court Judge Legrome D. Davis said it was one of the oldest cases before him, but there is at least one final hearing upcoming.
Along with seeking forfeiture, the government filed a motion for a declaratory judgement that the coins are American government property.
Because she inherited the box, Langbord said, she had not previously fully searched it, and only occasionally opened it to remove a piece of her mother's jewelry, uniquely styled and sought after by a customer, for sale at I. Switt, the Jeweler's Row store founded by her father.
"You never knew what was on the Assistant U.S. Attorney Nancy Rue asked on cross-examination.
"No, ma'am, I did not," Langbord said.
The coins were seized by the U.S. Treasury, but District Judge Legrome D. Davis ruled the government had to prove they had been stolen.
Langbord's appearance was a rare break after days of mind-numbing testimony about Mint records that government attorneys said accurately tracked every coin produced at the facility.
Berke contended otherwise, and in his closing said the complex records were incomplete and rife with errors, making it impossible for anyone to state with certainty that the coins were stolen.
In the 1940s, Switt was investigated by the Secret Service, which seized nine double eagles that he had sold to coin dealers after 1937. Switt said he had no recollection of how he had gotten those coins, and swore under oath that he had no others.
His daughter testified that while she had worked in her father's store from age 9, she had not been involved in purchasing goods and had never known he had the coins.