Roman Hat Mystery — Ellery Queen

Ellery Queen is again faced with a seemingly insoluble problem - how Monte Field, a shady and prosperous New York lawyer, was murdered by poison during the second act of a smash hit play in the crowded Roman Theatre.
The seats surrounding the victim's were all surprisingly empty, and the poison which killed him was tetra-ethyl lead - almost instantaneously fatal and yet concocted from ordinary and untraceable petrol!
The only clue was inexplicable - Field's top-hat was missing!
How were Field's crooked activities connected with the crime? Where did he keep his secret papers? Which of his many blackmail victims had been goaded to make the final protest?
Ellery Queen answered all of these questions and many more, purely by deduction, and without any concrete grounds for his conclusions or for a conviction - and how this evidence was obtained makes an exciting and brilliant story.

This was the first Ellery Queen story ever published.