Force play

[9thIsleZebra]

We had this play last night. A's ball on the A 5 yard line. A1 fumbles the ball there is a scramble for the loose ball and the ball ends up in A's end zone where A2 falls on it. The R calls the crew in and asks "Did anyone see who was the last to touch the loose ball BEFORE it went into the end zone?" I was one of the wing officials and I could not tell who touched it last, all I know is the ball ended up in the end zone and thus I would have ruled safety. The U said that a team B player muffed the loose ball into the end zone and the R, upon hearing this, ruled touchback, 1st and 10 for A. My question is, does it really matter who is the last to touch a loose ball (fumble) to determine who forced the ball into the end zone? Since a new force may be applied on a grounded loose ball, I always thought as long as the ball is moving, the original force.....A's fumble......is not spent, so the responsibility lies with A.