The National Transportation and Safety Board has released the results of its investigation into the 2017 gas explosion at Minnehaha Academy that killed two staff members.
The NTSB found that two workers with Master Mechanical were preparing to move a gas meter in the basement of the south Minneapolis school on the morning of Aug. 2, 2017.
They believed a plug valve connected to the meter to have been in the closed position, with the wrench that turns the valve "stuck" in a perpendicular position, which usually indicates it is closed.
However, the NTSB found after the blast that while the wrench was in the perpendicular "closed" position, the valve itself was open, and that it was the responsibility of the workers to confirm it was closed.
The report also notes that both workers – a father and son duo – didn't have the required licensing and training to be carrying out the meter move.
"It is customary to align the handle so that it is parallel with the piping when the valve is open and positioning the handle perpendicular to the piping when the valve is closed. However, confirmation of this is the responsibility of the crew working on the piping. This confirmation would entail removal of the handle and noting the position of plug assembly and viewing the indicator or witness mark on the body of the valve."
As the workers proceeded to remove the piping, the open valve caused natural gas to flow out, with the workers unable to shut it down.
They began to raise the alarm, as did a school maintenance worker who heard a "horrendous flow of air" and then noticed a strong odor of natural gas.
"As he exited the basement, he [the maintenance worker] made an announcement over his handheld radio that there was gas in the building and to evacuate immediately," the report states.
"As he made his radio announcement, he ran up the stairs and searched for occupants. Less than 1 minute later, the building exploded."
Killed in the blast was 47-year-old school receptionist Ruth Berg and 82-year-old custodian John Carlson, with nine others injured.
The NTSB did find that there was a "lack of detailed documentation that clearly established the scope of work to be performed" on the day the meter was being moved.
In a statement, CenterPoint Energy said it has reviewed its procedures regarding the relocation of indoor meters to outside buildings, and said it now writes a letter each year to mechanical contractors in Minnesota "to remind them of the dividing line between our equipment and the customer's equipment and to emphasize that contractors are not authorized to work on or operate our equipment."
It has also reached confidential settlements with the Berg and Carlson families.
"We were saddened by the tragedy that occurred at Minnehaha Academy, and our deepest sympathy continues to be with everyone who was affected," the statement said.