To paraphrase the old saying, anyone who lies to the FBI has an idiot for a lawyer.
As I've referenced before in other threads, I had the wonderful pleasure of being "interviewed" by the FBI -- thankfully, not as a target but also not as a mere bystander. Those bastages were most definitely NOT after the truth. They were after skins for the wall and merely tried to lead me to support the bullshot story they invented to make that happen. I was lucky enough to have a terrific lawyer (he had formerly worked for the independent counsel's office during Iran-Contra) and his prep for me was stellar.
In addition to the obvious of not lying to the FBI, he prepped me with information including studies of the human brain and how memory works. This includes the human brain's penchant for unconciously identifying two related, but disconnected, memories and inserting an honestly believed "memory" in between them to make a story more complete. During our prep, he taught me to pause, think, and consider whether something I thought happened was an actual memory or not, or rather whether it was something I was merely surmising based on other background knowledge. It was truly frightening when I realized how natural and how often my brain wanted to do that.
That said, the DoJ and FBI weren't happy with the hundred times I said, "I don't have an actual memory of that but would you like me to surmise what I think might have happened?", knowing full well that they had no interest in hearing my surmising. They got pissed and just quit and hauled me up before the grand jury.
Of course, dealing with the FBI is a lot easier when you didn't do anything wrong. Vappie was in the soup from the get-go, so his level of difficulty was a lot harder than mine!