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An unopened case of ice hockey cards found in a basement office has earned a family in Canada $3.1m USD ($4.2m CAD; £2.4m) at auction.

Inside there could be more than 20 "rookie cards" of Wayne Gretzky, one of the sport's greatest ever players.

The family who discovered the case hidden in their Saskatchewan office were "ecstatic" at the sale, an auction house spokesman said.

The winning bid was made by a Canadian who is choosing to stay anonymous.

With a 20% premium on top of the purchase, the final amount the Canadian bidder will pay is about $3.7m.

Heritage Auctions, which is based in Dallas, called the discovery of the pristine 1979-80 cards from the now-defunct O-Pee-Chee candy and trading card brand, "the greatest unopened find of the 21st Century".

That characterisation is based on the valuable trading cards - Mr Gretzky's rookie card, in particular - that could be inside. Rookie cards are the first that depict an athlete as a professional player, and many collectors will pay a premium for them.

An individual Gretzky card can fetch anywhere from a few thousand to hundreds of thousands of dollars. Heritage Auctions sold one 1979 O-Pee-Chee card for $3.75m three years ago - a record.

Heritage Auctions said in its lot description that because there are 396 cards in the set, there could be about 27 physically pristine Gretzky rookies inside the brown cardboard case.

"And, I mean, there's no guarantees, but it would be a pretty big statistical anomaly" for it to deviate greatly from that number, said Jason Simonds, a sports cards specialist at Heritage

The family who had discovered the case "hosted a small viewing party with close friends to watch the bids come in", Mr Simonds said.

The dusty case was discovered when the father was clearing out his office in Regina with the help of his son, Mr Simonds told the ´óÏó´«Ã½.

The father was an "old-school collector" who had bought the sealed case years ago intending to open it and build sets of cards to sell but simply never got around to it, he said. The family have asked to remain anonymous.

Cases like this one, which contain 16 boxes with 48 packs per box and 14 cards per pack and over 10,000 total cards, were originally intended to be sold to large stores who would open them and sell the individual packs to customers.

Before going to auction, all 16 wax boxes within the larger box were authenticated.