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 10-page report Monday, Dec. 2,on the explosion at Minnehaha Academy that killed two people, injured nine and caused $30 million in damage at the private Christian school’s upper campus, which overlooks the Mississippi River just south of the Lake Street-Marshall Avenue Bridge between Minneapolis and St. Paul.

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 in the basement of the school’s original century-old building. They told investigators that they believed the gas valve was in the “off” position, but evidence recovered from the debris showed that an internal valve was still open. Gas leaked from the valve, causing the explosion and complicated fire suppression.

Neither of the two men moving the meters had the proper certification for the job. CenterPoint, which contracted Master Mechanical, says it changed its procedures shortly after the accident.

“We appreciate the thorough investigation conducted by the National Transportation Safety Board (NTSB) and respect the findings contained in the board’s final report,” Master Mechanical said in a statement. “Throughout this entire difficult time, we have cooperated fully with the NTSB. We continue to keep in our thoughts all those impacted by the incident.”

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Crew not properly trained

A father/son team was hired to install piping to support the relocation of gas meters from the basement of the building to the outside, the report said. Two new meters mounted on an exterior wall were ready for the connection. The meters were located in a “utility bunker,” which was an extension of the basement spaces constructed beneath a ground-level concrete slab that extended out from the west basement wall of the building. It was accessed by a basement door from the boiler room.

The Master Mechanical on-site work crew consisted of a field foreman and a construction helper; the field foreman was the father of the construction helper.

The foreman was trained as a journeyman/pipefitter and had worked at Master Mechanical for about eight years. Although he was licensed to meet state and local requirements, he was not qualified to work on this type of piping. His son was a part-time employee and was not trained to any pipefitter classification level. He was working on the gas piping immediately preceding the explosion, the report said.

Valve opened or closed?

The two men told investigators that prior to the explosion, they had encountered a valve in which the wrench was stuck in the closed position and could not be turned. The son said he knew the valve was closed, because the wrench was positioned perpendicular to the valve piping.

The foreman also stated that the valve was closed and that it was safe for his son to begin disassembling piping downstream of the valve.

Investigators were unable to determine how the valve wrench became stuck in that position. Pre-accident photos confirmed that the perpendicular position would indicate that the valve was closed.

After the accident, when the valve was disassembled, investigators found that the internal part of the valve was in the open position.

They concluded that whether the outer wrench was in the proper position or not, it was the responsibility of the crew to confirm that it was indeed closed.

'Horrendous flow of air'

Believing that the valve was closed, the construction helper began to dismantle it.

At that point, a school maintenance worker located on a floor above the basement heard a “horrendous flow of air,” and he immediately went to investigate the strong odor of natural gas and the loud noise coming from the basement, the report said. As he exited the basement, he made an announcement over his handheld radio that there was gas in the building and to evacuate immediately. As he made his radio announcement, he ran up the stairs and searched for occupants. Less than one minute later, the building exploded.

A free flow of natural gas continued to burn beneath the debris pile of the collapsed structure, making it difficult for firefighters to put out the fire.

CenterPoint closed the valve at 11:18 a.m., and the fire was extinguished.

Collateral damages

The estimated total property damage resulting from the explosion and fire was about $30 million. Killed were receptionist Ruth Berg, 47, and custodian John Carlson, 82. Nine others were injured.

School was out for the summer, but some programs were in session. About 42 staff members and students were evacuated from the school immediately before the explosion.

Five lawsuits were filed more than two years after the accident.

The lawsuits say CenterPoint and Master Mechanical knew their work at Minnehaha Academy was “hazardous and abnormally dangerous” but “failed to inspect and close shut-off valves” upstream from a gas meter.

The plaintiffs include the school’s president, who says she suffered a traumatic brain injury; a soccer coach, who lost a leg; and three other employees, who say they suffered concussions and post-traumatic stress disorder.

As it rebuilt, Minnehaha Academy moved upper school classes to the former Brown College campus in Mendota Heights. The new building, funded with a $50 million capital campaign, opened last fall.