[hackersword]

>Although the FBI agents told me I could leave whenever I wanted, when the door closed and they started questioning me, it certainly didn’t feel that way.

I know easy to say in hindsight and safely behind keyboard , but you need to call them on that at the very first instance. Before any other words have left their mouth.

You realistically may NOT be "free to leave"; and if they demonstrate that by physically barring you from the door you need to repeatedly ask them if you are free to leave or being detained. Make sure the "lie" is obvious.

At that point, it is even more of a cue to shut your damn mouth.