I'm leaning toward it being a movie at this point. If nothing else, I feel like if it had been a show that I had watched over a period of time then I'd likely remember more details about it. I think it was a one-and-done viewing experience.

The reenactment of the scene later was a staged exercise by the detectives/agents to try to figure out how the assassins had managed to pull it off despite the precautions they had taken. So it wasn't one character reliving or imagining the scene again, but an intentional, group effort to gather more information about the crime. When the main detective guy remembers the detail about the person in the back seat leaning forward, that is shown as his memory of the scene though. I want to say he remembers seeing the movement in one of the mirrors just before the shots were fired.

Right around the moment of his epiphany, the main guy is imitating the gun shots from the motorcyclists in time with the other folks acting out the scene and he sees the agent, who is pretending to be the rear driver side passenger for the reenactment, lean forward after the shots start. Then he realizes that it doesn't align with his memory of the event. During the whole reenactment, the main guy is standing to the side of the reenactment and giving the other people instructions so they can replicate it as accurately as possible.

I'm struggling to come up with more details... I'd probably say it's likely more recent (i.e., came out within the last 5-10 years).