Jake,
It was Ruby behind the fence and murdering JFK. That's why he also killed Oswald because he wanted to kill two birds with one stone but spare the grief of Jackie coming back to Dallas to testify.
Here is the exact moment when the fatal shot was fired. As you can see, he has that smug Jack Ruby smirk on him as he lowers his rifle to admire his handiwork. He knew he scored big on the fatal shot.
And here is a close up of the photo further verifying it was Jack:
And here it is again closer up. There's no doubt. He didn't even bother changing fedoras as it's the same one he wore Sunday when he murdered Oswald on live TV: