The Commerce Department and the Navy led the investigations into the Hindenburg disaster. However, the Federal Bureau of Investigation also looked into the matter even though it technically had no jurisdiction. President FDR had asked all governmental agencies to cooperate in the investigation. The FBI files released about the incident through the Freedom of Information Act are available online.
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The theories of sabotage began to surface immediately. People believed that maybe the Hindenburg had been sabotaged to harm Hitler's Nazi regime. The sabotage theories centered on a bomb of some sort being placed aboard the Hindenburg and later detonated or some other sort of sabotage performed by someone on board. Commander Rosendahl of the Department of Commerce believed that sabotage was the culprit. (See p. 98 of Part I of the FBI documents.) According to a Memorandum to the Director of the FBI dated May 11, 1937, when Captain Anton Wittemann, the third in command of the Hindenburg, was questioned after the tragedy he said that Captain Max Pruss, Captain Ernst Lehmann and he had been warned of a possible incident. He was told by the FBI Special Agents not to speak of the warning to anyone. (See p. 80 of Part I of the FBI documents.) There is no indication that his claims were ever looked into, and no other evidence arose to support the idea of sabotage.
Some people pointed to a possible mechanical failure. Many of the ground crew later interviewed in the investigation indicated that the Hindenburg was coming in too fast. They believed that the airship was thrown into a full reverse to slow the craft. (See p. 43 of Part I of the FBI documents.) The speculation arose that this may have caused a mechanical failure which sparked a fire causing the hydrogen to explode. This theory is supported by the fire at the tail section of the craft but not much else. The Zeppelins had a great track record, and there is little other evidence to support this speculation.
The next theory, and probably the most outlandish, involves the dirigible being shot from the sky. The investigation focused on reports of a pair of tracks found near the back of the airfield in a restricted area. However, there were numerous people on hand to watch the amazing event of the Hindenburg landing so these footprints could have been made by anyone. In fact, the Navy had caught a couple of boys who had sneaked into the airfield from that direction. There were also reports of farmers shooting at other dirigibles because they passed over their farms. Some people even claimed that joy seekers shot down the Hindenburg. (See p. 80 of Part I of the FBI documents.) Most people dismissed these accusations as nonsense, and the formal investigation never substantiated the theory that the Hindenburg was shot from the sky.
The theory that gained the most popularity and became the most widely accepted involved the hydrogen on the Hindenburg. Hydrogen is a highly flammable gas, and most people believed that something caused the hydrogen to spark, thus causing the explosion and fire. In the beginning of the investigation, the idea arose that the drop lines carried static electricity back up to the airship which caused the explosion. However, the chief of the ground crew denied this claim by the fact that the mooring lines were not conductors of static electricity. (See p. 39 of Part I of the FBI documents.) More credible was the idea that the blue arc seen at the tail of the airship just before it burst into flames was lightning and caused the detonation of the hydrogen. This theory was substantiated by the presence of the lightning storms reported in the area.
The hydrogen explosion theory became accepted as the reason for the explosion and led to the end of commercial lighter-than-air flight and the stalling of hydrogen as a reliable fuel. Many people pointed to the flammability of the hydrogen and questioned why helium was not used in the craft. It is interesting to note that a similar event happened to a helium dirigible the year before. So what really caused the end of the Hindenburg?