Zangara arrived in Miami from Paterson, New Jersey weeks before FDR’s visit to the city. Intending to travel to Washington to shoot Hoover, Zangara elected to remain in Miami and kill Roosevelt instead. According to FBI memoranda, he bought a cheap 32 caliber pistol at a local pawn shop. (FBI memo to J. Edgar Hoover from Agent C. D. McKean) At Bayfront Park, Zangara unloaded all five shots, 25 feet from Roosevelt who was just completing his speech. Several in the audience were wounded but only Mayor Cermak of Chicago would eventually die.
You can find more information on the prison as well. This was the shortest time from crime to execution.