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When it comes to nuclear weapons, basically everything is classified.

When the FBI raided Mar-a-Lago in search of sensitive documents Monday, it was looking for classified information related to nuclear weapons, a source familiar with the investigation told the Washington Post. Trump denied the allegation on Truth Social, calling it a hoax and comparing it to accusations of Russian meddling in the 2016 election. The problem is, because of the way nuclear secrecy works in the U.S., it’s possible Trump took something without realizing it was classified. Presidents have done similar things before with regards to nuclear secrets.

I reached out to several nuclear weapons experts, and they all told me the same thing: They had no idea what it was, and the list of possibilities was enormous. The category of “classified documents relating to nuclear weapons” is so broad as to be meaningless.

“I've seen other experts speculating it could be anything from the ‘biscuit’ that held the nuclear codes during his presidency (and that would have been changed when Biden assumed office), to information about another country's nuclear program in the form of briefing materials or other documents, to information about design or basing,” Emma Claire Foley, a senior associate in policy and research at Global Zero, a nonprofit that seeks the elimination of nuclear weapons, told me. “As things stand, there's no way of knowing, and there's a huge range of things it might be.”

Jeffrey Lewis, director of the East Asia Nonproliferation Project at the Middlebury Institute of International Studies, concurred. “It could be anything,” he said. “That document could contain information that you would find totally unremarkable but still be marked restricted data, known as Q.”

Q Clearance refers to Top-Secret Restricted Data related to America’s nuclear programs. It’s also where the QAnon conspiracy gets its name. “I once gave a talk about Iran’s centrifuge program and the person next to me kept whispering, ‘That’s Q,’ thinking that I was disclosing ‘information related to nuclear weapons,’” he said. “I felt bad when I told her that I didn’t have a clearance but it was nice the government was also aware of the issues that Iran was having with its enrichment program.”

One of the problems with narrowing down what the FBI was looking for is the nature of how the U.S. handles information related to its nuclear program. According to Alex Wellerstein, a historian of science and author of the book Restricted Data: The History of Nuclear Secrecy in the USA, anything related to nukes in the U.S. is classified by default.

According to Wellerstein, the Senate committee drafting the Atomic Energy Act in 1946 got worried about losing the secret of the bomb.

“The problem is, the same committee did not want to give the federal government the power to classify the entire world of science and technology. They feared too much classification, AND too little classification,” he said on Twitter. “So to square this circle they created a concept called ‘restricted data,’ which was defined as essentially all information about nuclear weapons and nuclear power that had not been removed from that category explicitly.”

Wellerstein also told me he wouldn’t speculate on what Trump could have taken, but he did say it’s not the first time a president has run afoul of the FBI over nuclear secrets. An FBI memo from November 1956 showed the FBI fretting over what to do about former President Harry Truman.

“In August of 1956, during the discussion at the Democratic National Convention regarding the Democratic platform, Truman made the statement that the first atomic bomb contained [redacted] of fissionable material,” an FBI memo said. “[Redacted] advised that this information had been checked thoroughly by the [Atomic Energy Commission] people, and that its present classification is ‘Secret—Restricted Data.’”

Wellerstein had the whole story. “In November 1956, the FBI became aware that Truman was apparently telling people how much fissile material was in the atomic bomb,” he said. “After the Trinity test, Truman was given a short initial report on it, at Potsdam. According to the people there, he sort of read it out loud as he walked around the room, many times, triumphantly. Throughout his life, you can find him still quoting bits from that report, like he burned it into his memory.”

Wellerstein said one piece of the report that amazed Truman was that the bomb used only 13 and a half pounds of plutonium. “And there's something about that ‘thirteen and a half’ but he wrote it down at Potsdam, and it HAS to be what the FBI report is about,” he said. “Harry Truman was telling people about the atomic bomb and spat out that phrase without probably having the slightest clue it was classified.”

On Twitter, Wellerstein pointed out that the AEC doesn’t contain provisions for a president to declassify Restricted Data. “It’s a different category of law and classification altogether,” he said. “But this is real bleeding-edge of presidential powers and classification law. I have certainly never seen it discussed in the long history of nuclear secrecy in the USA.”

This isn’t the first time people have raised concerns about how Trump handles nuclear secrets. Allegations of mishandling plagued him during much of his presidency. In 2019, whistleblowers raised concerns that Trump was trying to transfer sensitive nuclear data to Saudi Arabia. In 2019 Trump was also the first U.S. president to confirm that U.S. nuclear weapons are housed at an air base in Turkey, long considered an open secret, and tweeted out a classified satellite photo of an explosion at a space launch facility in Iran, despite pushback from aides. So far he has not faced consequences for any of these actions.

The nuclear secrets the FBI is looking for could be innocuous, or they could be world-shattering. The possibilities are so large that there’s no way to know. Historically, the consequences for stealing nuclear secrets in America are pretty dire. Early Trump lawyer and mentor Roy Cohn made his career by prosecuting Julius and Ethel Rosenberg for stealing nuclear secrets and passing them to the Soviet Union. Cohn won the case, and the Rosenbergs were executed in the electric chair.