Touchback or Saftey???

[ajkottke]

Team A has the ball 1st and Goal from B's 3.

A3 fumbles the ball into B's endzone. B1 picks up the fumble, runs latterally for 5 yards and then throws the ball out the back of the endzone.

Is this a touchback or saftey? Why? (NFHS Rule)

A3's fumble forced the ball over the goal line.
B1's backward pass forced the ball over the end line.

Regards,
AJ