A copy of the first Superman issue, unearthed by three brothers cleaning out their late mother's attic, netted $9.12 million this month at a Texas auction house which says it is the most expensive comic book ever sold.

The brothers discovered the comic book in a cardboard box beneath layers of brittle newspapers, dust and cobwebs in their deceased mother's San Francisco home last year, alongside a handful of other rare comics that she and her sibling had collected on the cusp of World War II.

She had told her children she had a valuable comic book collection hidden away, but they had never seen it until they put her house up for sale and decided to comb through the basement for heirlooms, said Lon Allen, vice president of comics at Heritage Auctions. The brothers uncovered the box of comics and sent a message to the auction company, leading Allen to fly out to San Francisco earlier this year to inspect their copy of "Superman No. 1" and show it to other experts for appraisal.

“It was just in an attic, sitting in a box, could have easily been thrown away, could’ve easily been destroyed in a thousand different ways,” Allen said. “A lot of people got excited because it’s just every factor in collecting that you could possibly want all rolled into one.”

The "Superman No. 1" comic, released in 1939 by Detective Comics Inc., is one of a small number of copies known to be in existence and is in excellent condition. The Man of Steel was the first superhero to enter pop culture, helping boost the copy's value among collectors, alongside its improbable backstory, Allen said.

The previous record for the world's most expensive comic book had been set last year, when an "Action Comics No. 1" — which first introduced Superman to the world as part of an anthology — sold for $6 million. In 2022, another Superman No. 1 sold for $5.3 million.

A small, in-house advertisement in the comic book helped experts identify it as originating from the first edition of 500,000 Superman No. 1 copies ever printed. Allen estimates there are fewer than 500 in existence today.

The copy was not given any special protection, but the cool Northern California climate helped preserve it, leaving it with a firm spine, vibrant colors and crisp corners, according to a statement from Dallas-based Heritage Auctions. The copy was rated a 9.0 out of 10 by comics grading company CGC, meaning it had only the slightest signs of wear and aging.

The three brothers, in their 50s and 60s, did not wish to be identified due to the windfall involved nor did the buyer of the comic book, according to the auction house.

“This isn’t simply a story about old paper and ink,” one brother said in a statement released by the auction house. “This was never just about a collectible. This is a testament to memory, family and the unexpected ways the past finds its way back to us.”

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Brook is a corps member for The Associated Press/Report for America Statehouse News Initiative. Report for America is a nonprofit national service program that places journalists in local newsrooms to report on undercovered issues.