This is just at first contact. The GT player wraps up the Duke player and pulls him into(or further into) the end zone. I can't tell from that angle if the ball is on top of or in front of the goal line. Gary Patterson (who made the call) is standing on top of the goal line, so he should have a good angle. If a defensive (kick coverage) player makes contact with a ball carrier and pushes him backwards, the ball is spotted at the point of contact. Even if the ball carrier was moving in a backwards direction when he was contacted.
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I thought it was a safety during the game, and I still think it was a safety. However, I have not seen an angle that I can draw a line on and say for absolutely sure that the ball broke the plane of the goal line.