(Minneapolis, MN) -- A National Transportation Safety Board (NTSB) report blames gas meter workers for an explosion at Minnehaha Academy that killed two people and left nine others injured in August 2017. The report found the explosion likely occurred when a pipefitting crew disassembled piping upstream of a gas service meter. Reportedly a two-person crew with Master Mechanical believed a "plug valve" in the gas line was shut off because the handle was perpendicular to the gas piping. But a post-explosion investigation determined that an internal shutoff in the valve was in the open position. The blast killed the school’s janitor, 82-year-old John Carlson, and receptionist, 47-year-old Ruth Berg. The estimated property damage totaled about 30-million dollars.