Roy Langbord had guessed that someone in his family might have hidden away a great treasure decades before, but not until his mother had him check a long-neglected safe-deposit box did he realize just how great it was.

Inside the box, opened in 2003, he found an incredibly rare coin, wrapped in a delicate paper sleeve. It was a gold $20 piece with Lady Liberty on one side, a bald eagle flying across the other and, at Liberty’s left, the four digits that made it so valuable: 1933.

The famous “double eagles” from that year were never officially released by the government. Only a few had ever made their way out of federal vaults, and only one had ever been sold publicly, in 2002. The price: $7.6 million.

And there were nine more of them in the safe-deposit box.

But after the Langbord family took the coins to the United States Mint to be authenticated in 2004, they got a rude surprise. The Mint said the coins were genuine and kept them.

The government claims that they are government property stolen from the Mint, most likely in the 1930s, by Mr. Langbord’s grandfather, Israel Switt, a Philadelphia jewelry dealer.