The FBI is federal law enforcement, so they chose not to act on it. But I think they believed it to be a local issue not a federal issue and did not get involved. Not everything requires federal intervention, unlike today where people are always looking for the Feds to get involved when something like that happens. I am not saying I agree with it, its a plausible theory that does have merit to consider when looking at the mindset. It should be noted, that event did lead Kennedy and Johnson to push for equal rights. So their lack of action help with securing the Civil Rights legislation.
