I want to have one of my characters, a 25-year-old white mail, draw the attention of the FBI and be investigated. It's not a detective novel, it's more or less a romance, but I want it to be realistic. I don't know much about investigations and so forth.
The suspect "TP" has a job buying multiple copies of a novel to boost it's rating on the New York Times best seller list. (this would have worked at one time, not any more, but close enough.) The author of the novel, "BD", becomes suspected of being subversive. The investigation turns up what TP has been doing. TP has written an unsuccessful novel of his own that he's been trying to sell for years. By sheer chance there are a number of similarities between the two books, going back to before the publication of TP's book. The investigator reasonably but wrongly believes that there is some more connection between TP and BD and that BD may give more information. Investigation can find no sign that the two have communicated (because they haven't).
Now what? I guess they would put a tail on him and discovers some mildly weird behavior. They can't find anything more. They decide that TD and BD are not communicating any more. Would they ask him in for questioning? If he refused that, what would they do? Would they go by the apartment to question him? There's not enough evidence to get a search warrant, I would think.