beam weapons.
One of the more controversial topics involving Nikola Tesla is what became of many of his technical and scientific papers after he died in 1943. Just before his death at the height of World War II, he claimed that he had perfected his so-called "death beam." So it was natural that the FBI and other U.S. Government agencies would be interested in any scientific ideas involving weaponry. Some were concerned that Tesla's papers might fall into the hands of the Axis powers or the Soviets.

Tesla's suite at the hotel New Yorker following his funeral, 1943
The morning after the inventor's death, his nephew Sava Kosanovic´ hurried to his uncle's room at the Hotel New Yorker. He was an up-and-coming Yugoslav official with suspected connections to the communist party in his country. By the time he arrived, Tesla's body had already been removed, and Kosanovic´ suspected that someone had already gone through his uncle's effects. Technical papers were missing as well as a black notebook he knew Tesla kept—a notebook with several hundred pages, some of which were marked "Government."
In 1952, Tesla's few remaining papers and possessions were released to Sava Kosanovic´ and returned to Belgrade, Yugoslavia where a museum was created in the inventor's honor. For many years, under Tito's communist regime, it was extremely difficult for Western journalists and scholars to gain access to the Tesla archive in Yugoslavia; even then they were allowed to see only selected papers. This was not the case for Soviet scientists who came in delegations during the 1950s. Concerns increased in 1960 when Soviet Premier Khrushchev announced to the Supreme Soviet that "a new and fantastic weapon was in the hatching stage."
Work on beam weapons also continued in the United States. In 1958 the Defense Advanced Research Projects Agency (DARPA) initiated a top-secret project code-named "Seesaw" at Lawrence Livermore Laboratory to develop a charged-particle beam weapon. More than ten years and twenty-seven million dollars later, the project was abandoned "because of the projected high costs associated with implementation as well as the formidable technical problems associated with propagating a beam through very long ranges in the atmosphere." Scientists associated with the project had no knowledge of Tesla's papers.
In the late 1970s, there was fear that the Soviets may have achieved a technological breakthrough. Some U.S. defense analysts concluded that a large beam weapon facility was under construction near the Sino-Soviet border in Southern Russia.

Suspected Soviet beam weapon installation
Here are the FBI files on Teslas documents.
http://www.pbs.org/tesla/ll/ll_mispapers.html