Was the security still managed by the military? I know it was all still very important information.
Id I understand what I'm reading correctly, the Armed Forces Special Weapons Project was formed to handle the military aspects covered by the Manhattan Project, while the AEC continued in the civilian direction.
I also know the FBI did extensive background checks for the AEC, but what about day-to-day? Were civilians in charge of keeping the laboratories secure? Or did soldiers still manage security for the civilian scientists like they had when Los Alamos first formed?
Please let me know if I've gotten anything terribly wrong in my base knowledge here, my usual historical knowledge falls about 1700 years short of this. And if you have any suggestions for books or documentaries on the AEC, especially its early operations, I'd appreciate that!
I'd also love any book or documentary recommendations on the general ramp up of the Cold War from the end of the war to Stalin's death. (I've already watched "1945-1953: From World War to Cold War).
Talk about a niche question! But fortunately I just wrote a book on this, more or less.
The AEC had a Security Division that was in charge of physical security. They were the civilians in charge of plant security, internal security (e.g., looking for spies and etc., separate from the FBI and whatnot), and maintaining control over the weapon cores.
The AFSWP was the part of the military that was originally tasked with dealing with questions of weapons delivery and use. They were not given access to fully-assembled weapons, however. Over the period of the 1940s through the 1950s, the "custody" issue was worked out in various ways (see here for an overview) but the long and short of it is that under Truman the military was not given physical access to working nuclear weapons. (They were, at times, given access to the non-nuclear components, but the AEC held onto the nuclear components, kept near air bases in at least three sites in the USA.) In 1951 Truman gave the military access to 9 nuclear weapons, which were to be stationed on an aircraft carrier near Guam, and not to be used without his express permission.
The FBI's formal role was for background investigations only. They were not in charge of any other form of security. They did not even give determinations on clearances; what they did was present their findings to the AEC, who had a board that would then decide, based on the FBI's report, whether they would get a clearance. Even this was a MASSIVE job for the FBI considering how many clearances needed to be processed, and one of the revisions to the Atomic Energy Act they pushed for in 1954 was to allow the somewhat more mundane clearance investigations (e.g., for the construction laborers at the sites) to be offloaded to the Civil Service Commission.
Of course, the FBI did investigate espionage cases as well, but this was generally not coordinated very closely with AEC Security. The FBI and the AEC had a tricky relationship, to put it mildly; both maintained their autonomy from one another. One of the interesting aspects of a recent article I wrote about an investigation into lost H-bomb secrets in 1953 is that the FBI found itself somewhat pulled between the AEC and the Joint Committee on Atomic Energy (the congressional oversight committee that had authority over the AEC), and decided to throw in their weight mostly with the AEC, because they thought the JCAE staff were unreliable. In practice these things could go in many different directions in the complex Washington, DC ecosystem. One point I frequently emphasize is that most of the time the AEC was politically weak, because it did not have natural political allies (other than scientists, who had an ambivalent relationship to it, and were themselves politically weak).
Generally speaking the military-civilian split was kept very "real" under Truman. Under Eisenhower the boundary became a bit more porous, but the military never provided security at AEC facilities to my knowledge.