Seventy-five years have passed since Japan bombed Pearl Harbor Dec. 7, 1941, yet one mystery still perplexes some historians.

Several strange advertisements appeared in The New Yorker Nov. 22, 1941, two weeks prior to the attack, TIME reports. The main ad‘s title was “Achtung, Warning, Alerte!” and showed a group of people gathered in an air raid shelter.

We hope you’ll never have to spend a long winter’s night in an air-raid shelter, but we were just thinking…it’s only common sense to be prepared. If you’re not too busy between now and Christmas, why not sit down and plan a list of the things you’ll want to have on hand…Canned goods, of course, and candles, Sterno, bottled water, sugar, coffee or tea, brandy, and plenty of cigarettes, sweaters and blankets, books or magazines, vitamin capsules…And though it’s no time, really, to be thinking of what’s fashionable, we bet that most of your friends will remember to include those intriguing dice and chips which make Chicago’s favorite game: THE DEADLY DOUBLE.

Six smaller ads appeared throughout the issue.

One showed a picture of two dice with the numbers 12 and seven; those numbers do not appear on standard issue dice.

During World War II, an intelligence officer told U.S. Navy transport pilot Joseph Bell many members of the intelligence community believed the ads to be a secret warning, “Pearl Harbor: From Infamy to Greatness” author Craig Nelson told TIME. The officer had reportedly been instructed to investigate the matter, but all leads turned out to be dead ends.

The game, “The Deadly Double,”and the company that produced it were said to have never existed.

Some suspect that the ads were meant to warn Japanese loyalists living in the U.S. and prepare them for war. Lasislas Farago, a former U.S. intelligence expert and military historian, reported in the 1960s that it might have been a message alerting Japanese agents abroad.

“It’s a mystery to this day,” Nelson said. “I don’t quite believe if it’s a warning, but if it’s not—what is it?”

An Ad That Predicted #PearlHarbor? ‘Deadly Double’ Pearl Harbor Mystery Wasn’t So Mysterious After All https://t.co/EYwl95A15d pic.twitter.com/PZPlGsR1a2 — History @ SHS (@HistoryAtSHS) December 8, 2016

The mystery may have actually been solved long ago.

The game did, in fact, exist and was reportedly invented by Roger Paul Craig, who was tracked down by Los Angeles Times columnist Chapin Hall five months after the bombing of Pearl Harbor.

Craig denied that the message was a warning to Japan. “Nothing travels as far and fast as a grossly inaccurate and malicious rumor,” he explained.

In a later interview conducted after Craig’s passing, his widow, E. Shaw Cole, revealed that she helped him write the ads.

The connection between the ads and the attack on Pearl Harbor, she said, was just “one big coincidence.” She also said she had no idea how her husband picked the numbers.

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