Nikola Tesla was an uncle of Sava Kosanovic, who was the Yugoslav ambassador to the U.S. at the time. Tesla's entire legacy was to be his. The U.S. government feared that he was also a Yugoslav spy and even considered arresting him to prevent him from inheriting Tesla's property.
After a U.S. court declared Kosanovic the rightful heir to his uncle's estate in 1952, Tesla's files and other materials were sent to Belgrade, Yugoslavia, where they're now housed in the Nikola Tesla Museum. But even though the FBI originally listed 80 suitcases under Tesla's estate, only 60 arrived in Belgrade. Marc Seifer, author of the biography Wizard: The Life & Times of Nikola Tesla, says "Maybe they packed the 80 into 60, but there's also the possibility that ... the government kept the missing boxes."