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The article links directly to the FBI Vault -

Clandestine communists

One suggestion was that the airship could have been destroyed by a secretive group of communists and anti-fascists. A declassified letter written by FBI special agent G N Lowdon highlighted remarks made in Communist Party of America newspaper, the Daily Worker. Lowdon said a particular article in the newspaper caught his attention after reports in the US press about the “possibility that the ‘Hindenberg’ was sabotaged”.

He claimed the Daily Worker carried an article a week before the disaster claiming that German seamen were being recruited to do “perilous underground work aboard giant Reich liners plowing between New York and Germany.”

Although Lowdon said he found no evidence of this, it raised the possibility that there were clandestine groups preparing to sabotage transport systems associated with Nazi Germany.

Acrobat theory

Commander Charles E Rosendahl, who was in charge of Lakehurst Naval Air Station on May 6, 1937, told FBI investigators he saw sabotage as a “logical cause”. The FBI notes that “due to various happenings that have been called to his attention” Rosendahl is “of the opinion” that the fire in the gas shaft was started by an individual.

One incident said to have irked Rosendahl is an account by Dr Hugo Eckner, manager of Luftschiffbau Zeppelin, who interviewed Hindenburg staff after the incident. Rosendahl then mentioned hearsay and suspicions about a passenger named Joseph Spah, who had been allowed to tend to his dogs in a freight room below the Hindenburg’s gas tanks. Spah was an acrobat, and according to Rosendahl could have conceivably used his abilities to reach the fuel area. The theory was not substantiated and came to nothing.

https://vault.fbi.gov/Hindenburg%20/Hindenburg%20Part%201%20of%204/view