overtime rule question

[jjb]

situation :NFHS rules .... 2nd / goal from the 7yl during the first offensive series of overtime . B 1 intercepts A's pass and has a clear sideline for an apparent game ending runback for a touchdown. WH blows the play dead and resets the ball to begin the 2 offensive series.

Since I am a relatively new official who attended the game to watch the crew I was able to consult my rules book which I tucked into the game program. Under "Resolving Tied Games", in the first paragraph..( an overtime period may include only one offensive series of downs if the defensive team scores a safety or touchdown. Also 8-1 : if the defensive team scores a safety or touchdown, the game is ended.

However I also read in 5-1-1...if the defensive team gains possession, the ball becomes dead immediately and the offensive teams series of downs is ended.

My question is ; How can a defensive team score a touchdown ? A 93 yard backward fumble or pass recovered in A endzone ?