The skin
An ex-NASA engineer, Addison Bain, recently studied the disaster to try to prove that hydrogen was not the cause of the fire.
A 60 year old sample of the mateial used to cover the Hindenburg was tested. It was still highly combustible even after that length of time! It also combusted when subject to an electrical discharge, what happened in 1937.
It seems from hand written letters from the time, and the modifications to the Graf Zeppelin, that the real cause of the disaster was known, but it was hidden from the public to maintain the reputation of the designers.