A popular conspiracy theory holds that Tesla invented a free energy device, but that the powers that be prevented him from publishing it, because it would hurt their investments. Of course, free energy as such does not existfree energy as such does not exist, and literally speaking, this conspiracy theory is easy to refute, although one might still claim he referred to something else and the energy is not literally free, just very abundant.
One element of those conspiracy theories, however, is the claim that his documents were confiscated and classified shortly after his death. My question is not about free energy, but rather:
Does or did the US government, CIA, FBI, or some other agency, confiscate documents by Nikola Tesla and keep those classified to this date?
Some sources where related claims are made. Only the 2nd source below (PBS) is remotely reliable, but that one too cites no sources.
Is there any reliable source confirming or refuting this claim? If true, one element of the conspiracy theory — they're hiding something — is actually true, and even if the thing being hidden is not free energy, it might be something else that harms powers that be.