The preview for TWA Flight 800 tells a seductive story, and you should be skeptical.
Why would the FBI cover something like that up? Unclear. It was an election year, but it's unclear how that would have affected the Clinton-Dole race, nor is it clear if the FBI is into taking sides. Maybe an attack would have necessitated going to war, and the U.S. didn't want to bother. Or maybe there isn't a coverup, maybe a plane just crashed seventeen years ago.
Epix isn't in the conspiracy theory business, or at least they weren't. Most of their previous documentaries are entertainment related. That's a point in their favor in terms of legitimacy (conspiracy theories are a big business now), but they'll need more. This documentary makes an extraordinary claim, and that requires extraordinary evidence. For this to work, they need to show why the FBI would want to cover up an attack (doable), that they could (harder) and that they did (hardest). For those who watch TWA Flight 800, come in with an open mind and a skeptical approach, and know that there is good money to be made in conspiracy theories these days.