The general principles governing control of information were simple, General Groves explains (in Now It Can Be Told: The Story of the Manhattan Project):
First, nothing should be published that would in any way disclose vital information. Second, nothing should be published that might attract attention to any phase of the project. Third, it was particularly important to keep such matters out of any magazine or newspaper that was likely to be read by an enemy agent or by anyone whose knowledge of scientific progress would enable him to guess what was going on.
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We were only too aware that the piecing together of bits of published information is a prime source of knowledge to every intelligence organization.
It was in order to prevent speculative articles as well as the publicizing of any of our efforts that the press and radio had been asked to avoid the use of certain words, such as “atomic energy.” Certain decoy words, such as “yttrium,” were included in the list to camouflage its real purpose. This was a step we did not want to take, for it automatically pointed out to the press that the government was interested. However, Howard insisted that we simply had to do it if press security was to be maintained. Most reluctantly we agreed. As it turned out, it was a very wise move and an absolutely essential one.
We wished, too, to avoid any widespread mention of such places as Hanford or Oak Ridge and all mention of Los Alamos, as well as any reference to the MED. We also did not want any mention of my name that might arouse the interest of a foreign agent in my activities. Yet to have banned all reference in the near-by papers to Oak Ridge or Hanford would have been neither practical nor desirable, for it would only have tended to attract attention locally. We did try to keep Los Alamos entirely out of the news, but the Knoxville papers were permitted to carry items—mostly in the nature of social notes—about employees and events at Oak Ridge, though nothing, of course, that would help the average reader determine the purpose of the project or its importance. The same leeway was given to the papers close to Hanford.
We did have several unfortunate security breaks, but none of them, so far as we could ever find out, attracted any particular interest. The one with the worst potential for damage was a radio program that discussed the possibilities of an atomic explosion. The script for this had been prepared for the regular news reporter on a network program; he himself had had nothing to do with writing it. Unfortunately, in order to meet his travel schedule, he delivered it from a small affiliated station, where apparently it had not been reviewed to make certain that it did not violate press censorship rules.
From all that we could ever discover, there had been no deliberate breach of security. The information on which the talk was based came from a scientist who was not connected with the project in any way but who evidently had an inkling of what was going on, gleaned, we thought, from some of the project’s scientists at the large laboratory in his city. The actual text was written for the reporter by a friend of the scientist. There was never any question in my mind but that the reporter delivered it in good faith. The failure of the radio station to stop it was attributable to plain carelessness.
Another incident that concerned us greatly was the appearance in a national magazine of an article hinting at the theory of implosion. While it did not violate any rules, it was most disturbing. A thorough investigation indicated that it resulted from the work of an alert and inquisitive reporter in another country.
There was one unfortunate happening not too long before the bombing, when a Congressman, in discussing an appropriations bill, commented on the importance of the Hanford Project. This item was picked out of the Congressional Record and was republished in a newspaper without any comment. I could never disabuse myself of the feeling that this newspaper did it with the deliberate intent of letting me know that our security prohibitions were not so effective as we thought.
Journalists are spies. The only difference is who pays them and where they are trained.