The FBI released in PDF form its file on the investigation they did into the murder of Leon Trotsky. Its 914 pages with some but not many results. The quality of reproduction is bad enough that some of it is unreadable. There is quite a bit of black out, not more than 5% maybe, but those could be the most interesting, who knows. It seems to be mostly hiding identities of sources, the same sources that are sometimes obviously planting false information. What is readable never builds a case, if the case is to resolve Frank Jacson's true identity or uncover his accomplices in the United States. The file was renamed to Jacques Mornard, still not his real name. The FBI wanted to trace his contacts in the United States and reveal Russian spies, traitors and accomplices. I am open to the theory that this is not the real file, this is the discard file, or after war started with Hitler they really didn't want to investigate Stalin. They never uncover anything about Jacson until years later when the book "Murder in Mexico" by General Salazar and Julian Gorkin comes out. The FBI photocopies pages of the book for the file. They run down false leads. They either do not interview obvious people or they are using them unseen to the public as informants. Anonymous letters are followed up. They show his mugshot to dockworkers and several of them say I know him.
They had the name of an accomplice. The trail obviously starts with Ruby Weil, born in Indiana, living in New York, who introduced Sylvia Ageloff to Jacqes Mornard. Not once do they suggest finding her or interviewing her. Sylvia, the girlfriend for two years, essential to help Jacson get close to Trotsky, is held and formally accused of murder in Mexico. She was released after 4 months, not necessarily exonerated, and returned to New York. Regardless of her guilt or innocence, she is the best witness to the assassin. She is not interviewed for a year and a half, then only one time, by an agent who has no other information and just has to take her word. In fact none of the American socialists who interacted with Jacson were interviewed. They weren't investigating the murder, that was Mexico's jurisdiction, but in the end they didn't make much progress.
Of interest is the mention of the name Henry Schnautz in a memo from J. Edgar Hoover in June 1942.
Sylvia Ageloff was the girlfriend of Trotsky's assassin. She said she was unaware that her handsome boyfriend was actually plotting to kill Trotsky. Her notoriety caused her two years later to live in New York using her mother's name and quietly teach a kindergarten class. The FBI tracked her down in spring 1942, hoping for some new information. The report of her interview mentioned that when news of the assassination came, she was having dinner with a Trotsky guard named Otto Schuessler. That is the page 10 (reproduced here - also page 11) that Hoover refers to in his memo. The sharp-eyed Hoover also has another case before him, Amtorg Trading Corporation Espionage, that also mentions two Trotsky guards, Otto Schuessler and Henry Schnautz. These men, according to the report, were in the United States in February and possibly still in New York. This information comes from an unnamed man in the Communist party who is a friend to a man who works in the Amtorg Corporation who tells an unnamed informant. The Amtorg Corporation was a Soviet trade company, and a known front for Soviet spies, which Hoover knows because he is investigating it.
The FBI apparently does not realize there is a war between Stalin and the Trotskyists. Information about Trotskyists that comes from the same source that killed Trotsky apparently does not trigger a consideration of motive. The report goes on further to say that this unnamed third hand source informs the FBI that these two men were involved in the Rubin case, and the State Department knows all about it. This case involved the movement of Russian spies, in 1938, who are by definition Stalinists, and the falsification of passports to facilitate their movement. An unnamed New York communist tells a man in a Russian spy front who tells an unnamed informant who tells the FBI that the information is good because the State Dept knows all about it. The New York agent that receives Hoover's memo simply summarizes the information in the Amtorg file for the benefit of being included in the Trotsky Assassination file.
Of course, Schuessler and Schnautz were both German names in 1942. At the time Henry Schnautz was said to be in the United States, February 1942, he was in Mexico wooing Esperanza, and even making a recording with her to send home to his folks. In 1938 when he was supposedly involved in an international spy ring, he was at home in Evansville raising farm animals and working as a carpenter. These are the same two men who previously in Sept 1941 in newspapers all over America had been wildly accused of plotting to kill Mornard while in the custody of the Mexican police. Trying to assassinate their prisoner would clearly have constituted a serious crime to the Mexican police, but the day after that report came out, Henry attends a movie with Esperanza. There was a Soviet misinformation campaign that obtained the names of Schnautz and Schuessler - German names associated with Trotsky when America was at war with Germany - and not only planted stories in the press but fed the FBI false information they took seriously.
vault.fbi.gov/leon-trotsky/leon-trotsky-part-3-of-5/at_download/file (relevant pages are 54, 64, 71)