This is from Atomic Bombing: How to Protect Yourself, published in 1950:

Of course, millions of us will go through our lives never seeing a spy or a saboteur going about his business. Thousands of us may, at one time or another, think we see something like that. Only hundreds will be right. It would be foolish for all of us to see enemy agents lurking behind every tree, to become frightened of our own shadows and report them to the F.B.I.

But we are citizens, we might see something which might be useful to the F.B.I. and it is our duty to report what we see. It is also our duty to know what is useful to the F.B.I. and what isn’t.

[…]

If you think your neighbor has “radical” views — that is none of your or the F.B.I.’s business. After all, it is the difference in views of our citizens, from the differences between Jefferson and Hamilton to the differences between Truman and Dewey, which have made our country strong.

But if you see your neighbor — and the views he expresses might seem to agree with yours completely — commit an act which might lead you to suspect that he might be committing espionage, sabotage or subversion, then report it to the F.B.I.

After that, forget about it. Mr. Hoover also said: “Do not circulate rumors about subversive activities, or draw conclusions from information you furnish the F.B.I. The data you possess might be incomplete or only partially accurate. By drawing conclusions based on insufficient evidence grave injustices might result to innocent persons.”

In other words, you might be wrong. In our system, it takes a court, a trial and a jury to say a man is guilty.