Federal N-T-S-B investigators are back at the scene of Wednesday’s natural gas explosion at Minnehaha Academy in Minneapolis that killed two staff members and injured nine. Spokesman Christopher Hart says, “Our mission is to understand not only what happened but, more importantly, why it happened, so that we can make recommendations to prevent it from happening again.” Federal officials mapped out their investigation strategy Thursday but hadn’t yet received the go-ahead that the building is safe to enter.