The National Transportation Safety Board’s investigation into a fatal gas explosion at a Minneapolis school two years ago blames the pipefitting crew as the “probable cause” of the blast that killed two and injured nine others.

The 10-page report detailing what happened before and after the Aug. 2, 2017 blast, says the contractors hired to move the school’s meters mistakenly thought a crucial valve was shut when it was actually open, spewing gas into the building as pipes were taken apart.

According to the NTSB, the two contractors working for Master Mechanical Inc., believed the valve to one of the meters was closed because the “plug valve” was turned parallel to the pipes.

However, the valve was open, and as the pipes were being dismantled, workers in the building smelled gas and heard a “horrendous flow of air” coming from the basement. Just seconds after an announcement was made to evacuate, the building exploded.

The report says the field foreman leading the Master Mechanical crew had not completed training required by Centerpoint Energy, which hired Master Mechanical to move the school's gas meters, and was not certified to work on a job with the scope of the Minnehaha Academy relocation project. His helper had a total work experience of less than eight months with Master Mechanical, and also had not completed the Centerpoint operator qualifications program.

Also noted was a “lack of detailed documentation” that clearly laid out the scope of the project.

According to the NTSB report, the service provider, CenterPoint Energy, did not have any workers on-site as the job was being performed. In the aftermath of the explosion, the utility changed its policies to require safety meetings between all parties involved ahead of work involving the relocation of gas meters. Other safety measures were also added.

CenterPoint Energy says it did not wait for the NTSB report to be completed to revamp its safety measures, adding that the company continues to be “fully committed to the safety of our employees, contractors, systems and the public.”

A spokesperson for Master Mechanical, Inc. released a statement after the findings of the NTSB were released:

“We appreciate the thorough investigation conducted by the National Transportation Safety Board (NTSB) and respect the findings contained in the board’s final report. 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."

A number of lawsuits have been filed in connection to the explosion, blaming CenterPoint Energy and Master Mechanical for not managing the meter relocation job with proper care.