I've used the Freedom of Information Act to get access to previously classified government documents a number of times over the years. I started making such requests during the immediate post-Reagan era and in those days the FBI in particular was moderately cooperative in its correspondence with you but otherwise extremely slow to respond. I got the FBI surveillance files on the novelist Mike Gold (Jews without Money etc.) but it took about five years. You have to be patient and persistent.
Fortunately, by now some documents, once released to one scholar or journalist, are made unclassified and available on the web. It's not as difficult as it used to be. What you get is often disappointing, though: entire pages of my Mike Gold materials are blacked out.
Those interested in trying their hand at FOIA requests need to consult two terrific web sites: one hosted by The Reporters Committee of Freedom of the Press (link) and another by The National Freedom of Information Coalition (link).
Contrary to conventional wisdom, sunshine is not a natural state.