Highly classified signals intelligence–intercepted radio and other electronic communications from adversarial governments–were among the information Donald Trump took to Mar-a-Lago, an official with knowledge of the search warrant targets told the Washington Post.
In the 15 boxes of illegally taken documents previously reacquired, officials found pieces of highly-sensitive signals intelligence, the very existence of which can tell foreign governments about the methods and sources of information collected by US national security agencies. Disclosure of signals intel can prompt targets to change their methods, which results in reduced effectiveness of American collection techniques in the future.
Although the source country of the information is unknown, such documents could be used in multiple ways, such as blackmailing a foreign government to hide embarrassing or criminal information, like the kidnapping and murder of a journalist, to obtain significant amounts of money for a family member. Or it could be sold to a foreign country seeking to seal up its communications network from American intel.