Van Alden was with the Treasury Department's Bureau of Prohibition.

It's silly to project the FBI of today back in time to the 1920s. It was a totally different beast. No one was expecting the Bureau of Investigation to go after organized crime in the 1920s.

Through the 1950s, the FBI was more of an internal security ministry than a law enforcement entity. It targeted radical anarchists, then Nazi spies, then the Soviet Union's spies in the United States, and the Communist Party USA.