As is known, after Tesla's death on the night of January 7, 1943, his nephew Sava Kosanovic came to his room at the New Yorker Hotel in the morning. He found the room where the body had already been removed. Going through his belongings it seemed to him that someone went through them because some technical documents were missing along with Tesla's notebook marked "Government".
Several trunks of papers, records and equipment were removed from his room by the Department of Justice's Alien Asset Custody Office. As FBI President J. Edgar Hoover claimed, "they had no knowledge that the papers had been photographed or that his men had done such a thing." What causes additional controversy is the fact that Tesla was a US citizen and it is incomprehensible why the Department of Justice's Alien Asset Custody Office could even be authorized to take over Tesla's assets and have jurisdiction at that time.
The FBI states that the analysis of the documents was carried out by John G. Trump, a RADAR scientist from MIT, with the assistance of senior officials of the Naval Intelligence. After a careful review of the documents, Trump states that he considers the papers safe for publication because they “do not constitute a hazard in unfriendly hands”. In his report, he also mentions information from Tesla's work related to the beam of particles, which he describes as insufficient for the manufacture of weapons, and if there is a possibility of performing the configured version it would be very limited in power.
According to the official statement by the FBI:
When Tesla died in New York in January 1943, his papers—which were thought to include plans for a particle beam weapon, dubbed a “death ray” by the press—were temporarily seized by the Department of Justice Alien Property Custodian Office (“alien” in this case means “foreigner,” although Tesla was a U.S. citizen). Despite longstanding reports and rumors, the FBI was not involved in searching Tesla’s effects, and it never had possession of his papers or any microfilm that may have been made of those papers. Since 1943, we have told a consistent story to all who have asked. Reports to the contrary appear to be based on an initial confusion of FBI agents with other government officials—especially Alien Property Office personnel. These rumors have long been repeated in biographies and articles on Tesla without double-checking the facts as reported in our files.
Here are available Official FBI files.