The movie J.F.K. is about a District Attorney from Louisiana who is trying to prove that Lee Harvey Oswald did not act alone in the assassination of John F. Kennedy. Jim Garrison the district attorney makes trips to Dallas where the assassination took place he along with his co-workers from his office would put together a plan of how the assassination happened.
JFK is a thriller with a political side to it, You know the film is politically influenced by the opening sequence to the film. The opening sequence starts with up beat diegetic music that contains drums, the fact the music is a drum beat shows its importance which could be represented as a salute to a political leader?.
The film title is JFK, so we know it will be about assassins which makes it a thriller, they don't need to emphasise on this point anymore as the audience will generally be able to make the link between the film and the happening of the event.