From the book:
Broken
The Troubled Past and Uncertain Future of the FBI
Richard Gid Powers
2004
In 1933 when it was decided to merge several bureaus. Hover did not want the 1200 “Dry Agents” from the Prohibition Bureau. Because they were hopelessly corrupt. And Hover did not want them corrupting or tarnishing his special agents.
So Hover setup a new Alcoholic Beverage Unit. To keep the Dry Agents from contaminating his men.
Guess what there called now.
DEA? no wait BATF?