The National Transportation Safety Board (NTSB) has determined that human error by unqualified contractors likely caused a natural gas explosion that killed two people and destroyed a building at a Minneapolis, MN, private school two years ago.

“The probable cause of the natural gas explosion at the Minnehaha Academy occurred when a pipefitting crew disassembled piping upstream of a gas service meter,” NTSB said in a report earlier this month. “Contributing to the accident was the lack of detailed documentation that clearly established the scope of the work to be performed.”

The two-person crew comprised the foreman, a licensed journeyman pipefitter, and his son, who was working as a part-time construction helper. CenterPoint Energy Minnesota Gas (CPEMG), which supplied gas to the school, had hired the crew as part of a project to move two natural gas meters from the building’s basement to its exterior.

The crew’s job on the day of the explosion was to relocate the pipeline that would connect the meters, which had already been relocated to the exterior, to the building. Before doing so, the workers had to first make sure that gas supply to the meters had been shut off.

NTSB interview statements from the two workers “indicated that, prior to the explosion, they had encountered a plug valve that was connected to and located immediately prior to the interruptible meter in which the wrench was ”stuck’ in the closed position (that is, the wrench could not be turned).”

The foreman therefore determined that the flow of gas to the meter was shut off, and that it was safe for his son to begin dissembling piping downstream of the valve.

However, on-site examination by the NTSB showed that the valve’s flow control component was, in fact, in the open position. As a result, the report said, “While workers were removing the existing piping, a full-flow natural gas line at pressure was opened. The workers were unable to control the release of the gas; thus, they evacuated the building and warned others to evacuate.”

The explosion, which killed a secretary and a custodial worker, as well as injuring nine other people, occurred during the evacuation.

The action by the pipefitting crew that likely caused the explosion occurred upstream from the interruptible meter, meaning that it occurred on Department of Transportation (DOT)-jurisdictional piping.

“Although the journeyman/pipefitter was licensed and trained to meet state and local requirements, he was not qualified to work on DOT-jurisdictional piping,” the report said.

CPEG’s parent, Houston-based CenterPoint Energy, said on Tuesday it had “implemented improvements to its procedures with the goal of preventing an accident like this from ever happening again.”

CenterPoint said it has “reached confidential settlement agreements with the families of the two school employees who died and with the person most seriously injured in the incident, but other litigation arising out of the incident is pending.”

In addition to reviewing and enhancing procedures and training, CenterPoint said, “We now send a letter each year to licensed 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 our equipment…”