Also the railroads. I know this because for many years, my Grandfather worked undercover to catch these spies. Many more were caught than the country ever learned about.
I didn't learn about his service until I was an adult. It was that secret.
Ahhh, the good old days when love of country, honor and ethics meant something.
The CIA, FBI, Generals in the military etc didn't "leak" top secret information to the press.
Maybe that is really why they are called the greatest generation.