When the giant German dirigible Hindenburg burst into flames over Lakehurst, New Jersey, on May 6, 1937, it left 36 dead, a pile of charred wreckage and one enduring mystery: What could have caused the horrific disaster?

Even before the ashes had cooled, rumors were flying. Three days later, the New York Daily News listed five leading theories:

1. Lightning

2. A backfiring motor

3. Ground crew negligence

4. A “cold spark”

5. An act of God

The following day, the paper added a sixth possibility: sabotage.

The list wouldn’t end there. In the ensuing days and weeks, international experts, well-meaning amateurs and assorted crazies bombarded investigators with theories of their own.

The Hindenburg had made its first flight from Germany to the U.S. a year earlier, in May 1936. This trip was intended to inaugurate its 1937 season, an event considered noteworthy enough to draw newspaper and newsreel photographers to Lakehurst. They would record unforgettable images of the ship bursting into flames and crashing to the ground as passengers and crew tried to leap to safety. From the first sign of fire to the Hindenburg coming to rest on the ground, the disaster lasted roughly 30 seconds.

The newspaper photographs appeared that night on front pages all over the world. The newsreel footage hit movie theaters the next morning.

German Chancellor Adolph Hitler received word of the disaster at his mountaintop retreat in Berchtesgaden, reportedly reacting with “stunned silence.”

Hugo Eckener, a German airship pioneer and head of the company that built the Hindenburg, first acknowledged the possibility of sabotage but then backtracked, saying that a stray spark probably ignited the ship’s highly flammable hydrogen gas.

Nazi Hermann Göring, the powerful head of the German air command, immediately dismissed the sabotage rumors, calling the disaster an act of God. “We bow to God’s will,” he said, “and at the same time, we face the future with an unbending will and passionate hearts to continue the work for the conquest of the air.”

Why the Nazis were so quick to write off sabotage, rather than twist the story for propaganda purposes, was a mystery in its own right. In his 2021 book Empires of the Sky, author Alexander Rose offers a theory. “If the Hindenburg had been blown up by evildoers,” Rose writes, “it would indicate that Hitler was not universally beloved, undermine the image of Germany as a placid and law-abiding society under Nazi rule, and give the enemies of the Reich, like the Jews and the Communists, hope that the regime was vulnerable.”

Unlike the Germans, Americans were under no such constraints, as contemporary newspaper accounts and declassified FBI files show. While the FBI didn’t formally investigate the Hindenburg incident, it assisted in the U.S. Commerce Department’s inquiry and became a contact point for citizens with theories to share.

While many correspondents suggested technical explanations for the disaster, the ones who favored sabotage showed the American imagination in full flower.

Many suggested that anti-Nazi elements were responsible. Those included communists, anti-Nazi Germans, Jews and Spaniards presumably angered by Germany’s support for the fascist leader Francisco Franco.

At least one correspondent suggested it was an inside job, that the Nazis themselves had blown up the Hindenburg for the insurance money.

The conspiratorially inclined also had a variety of theories regarding the means of destruction. An incendiary bullet fired from the ground was one possibility. (The FBI examined some suspicious footprints at one point but found nothing.) Another theory suggested a small plane had fired on the Hindenburg from above. One letter writer insisted it had been shot down by a New York City police lieutenant on the orders of Mayor Fiorello La Guardia.

The most common suspicion, though, was that a bomb had been hidden somewhere within the ship’s vast interior, ready to be activated by either a timer or a change in barometric pressure.

One sabotage theory focused on an American passenger named Joseph Spaeh (or Späh).

A German steward reported that he seemed suspiciously aloof and “unsympathetic to the airship travel.” It also happened that Spaeh was a professional contortionist and acrobat, useful talents for climbing around the ship’s interior structure and planting a bomb. At the urging of the base’s naval commander, the FBI checked out Spaeh and found nothing to incriminate him.

Spaeh would not be the only suspect. In a popular 1962 book, Who Destroyed the Hindenburg?, writer and military historian A. A. Hoehling accused a crew member of being the saboteur. Based on his own research, Hoehling believed that Eric (or Erich) Spehl, a 26-year-old rigger, had planted a bomb on board, supposedly egged on by his communist girlfriend. Hoehling admitted his case against Spehl was “circumstantial.”

In 1972, another author, Michael M. Mooney, also pointed the finger at Spehl in his book, The Hindenburg. This time, enterprising German reporters tracked down a woman they identified as Spehl’s former fiancée, who denounced the theory as “absolute madness.”

Mooney’s book, in turn, became source material for the 1975 film The Hindenburg, which made a similar case but changed the supposed saboteur’s name.

The real Spehl, meanwhile, was in no position to defend himself. He had been dead since 1937, a victim of the crash.

The U.S. and German governments each conducted inquiries into the crash, releasing their findings in July 1937 and January 1938 respectively.

Both concluded that atmospheric conditions that rainy evening had led to the disaster, although they differed as to the exact mechanism. The Americans suggested an electrical phenomenon called a “brush discharge” had most likely ignited leaking hydrogen, starting the fast-moving fire. The Germans favored the spark theory originally advanced by Hugo Eckener.

Even so, neither inquiry totally wrote off the possibility of sabotage. Nor have several generations of conspiracy theorists in the years since.

Although the general consensus among experts today is that the disaster was an accident, exactly what—or who—destroyed the Hindenburg may never be known with absolute certainty.