MINNEAPOLIS — Federal investigators have concluded that a deadly explosion at a Minneapolis school in 2017 happened because a pipefitting crew lacked proper training and wasn’t fully authorized to move a gas meter.

The National Transportation Safety Board released its report Monday on the explosion at Minnehaha Academy that killed two people and injured nine more.

Workers for the natural gas supplier CenterPoint Energy and Eagan-based Master Mechanical Inc. were moving meters on Aug. 2, 2017, when the explosion happened. School was out but some staff members and students were present.

The NTSB said the crew mistakenly believed a key valve was closed before they started disassembling the gas installation.

CenterPoint said it changed its procedures shortly after the accident. Master Mechanical said in a press release that it respects the board’s findings.