The Parallax View (1974) on Netflix

The official line: Senator Charles Carroll was assassinated by a lone gunman who also died at the scene, with no larger conspiracy involved in the assassination. Case closed. In the several following years after the assassination, six direct witnesses of the assassination - those that were featured in a published photograph - have themselves died, nothing seemingly tying these deaths together beyond the earlier event or deemed to be mysterious by authorities. However those four still surviving witnesses shown in the photograph, including television news reporter Lee Carter and Carroll's political advisor Austin Tucker, fear for their lives. Carter's newspaper investigative reporter colleague and former lover, Joe Frady, who was also at the event when the senator was assassinated, does not believe the conspiracy theory that Carter fears. Frady changes his opinion when more witness deaths occur. Frady, who has a history of irresponsibility in both his personal and professional life, decides to investigate. As he gets further into his investigative work, he does begin to think that the conspiracy theorists were correct, but that the conspiracy is much larger than even he could have imagined. Frady decides to go undercover, but there is race between Frady discovering the truth and the possible perpetrators discovering who Frady really is.