In 1937 Denis J. Mulligan was chief investigator of the U.S. Bureau of Air Commerce. Also in 1937, the German-built Hindenburg caught fire and crashed over Manhattan. 36 people were killed. Mulligan was the lead American investigator.
The cause of the crash is still unknown - although sabotage, lightning, a puncture, or a fuel leak have all been speculated as the cause. Mulligan had his own idea.
He had traveled on the Hindenburg just months before the crash. From this, Mulligan thought that faulty work from making the blimp longer and the mixture of air and hydrogen inside caused the fiery crash.
The Denis J. Mulligan papers at UW's American Heritage Center have documents recovered from the Hindenburg and correspondence from Mulligan's research.