If there was probable cause he'd be in jail. As for the grand jury sleuthing, they do not do that. Essentially, they do what the prosecutor wants them to do. And there's no reason to question the prosecutor in this case.
Correct me if I'm wrong (NOT all at once!!!) but once LE names someone as a suspect, they must "Mirandize" him/her. The suspect is no longer free to leave at any time and knows anything they say can be used against them in building LE's case. Until the person is named a suspect, he/she can leave at any time and are a "cooperating witness" or something. Anything they say can still be used in court, but the interviewee does not have to be notified of that fact until they are deemed a suspect by LE. Makes for a less adversarial relationship and seldom involves the interviewee bringing his lawyer along for company. More or less?