At about 5 p.m. on Thursday, July 4, 1940 a bomb explodes at the New York World’s Fair in Flushing, Queens, instantly killing two New York City police detectives and wounding five other men, two critically.

The New York World's Fair, which ran from April 1939 to October 1940 and took as its theme "Building the World of Tomorrow," attracted some 45 million visitors over its two seasons, making it the best-attended event of the first half of the 20th century.

The bomb, packed in a small canvas satchel, had been removed from the British pavilion an hour and a half earlier and taken to an open area at the edge of the fairgrounds. Attempting to determine whether it actually held a bomb, one detective cut a small strip from the satchel, revealing sticks of dynamite inside. Seconds later, it blew up.

An electrician in the building first noticed the satchel on Wednesday and assumed it had been left behind by a fellow worker. But when he saw it again on Thursday afternoon, he could hear faint ticking. According to a New York Times report, he carried the ticking satchel through the crowded building to his boss’s office, after which the two of them brought it to the pavilion’s head of security, who happened to be in the Magna Carta Room, where a priceless copy of the historic document was on display. He, in turn, called in his boss for a listen. At that point, they decided to alert police.

In retrospect, the lack of urgency seemed surprising. The pavilion had received an anonymous bomb threat earlier in the week and the police had assigned detectives to circulate among the crowds of visitors there. In addition, England was at war with both Germany and Italy, each of which had sympathizers, and possibly saboteurs, in the still-neutral U.S.

In coming days, police questioned more than 100 possible suspects, including members of fascist and pro-Nazi groups. They also began to investigate workers at the British pavilion, on the theory that it might have been an inside job. New York Mayor Fiorello LaGuardia offered a $25,000 reward, then a considerable sum, for information leading to the arrest and conviction of the perpetrators. On July 10, the police announced that they had put an unprecedented 1,500 detectives on the case.

Even so, the case would go unsolved. Whoever planted the bomb—and whatever their motive might have been—remains a mystery.