FARMINGTON — The collapse last week of steel joists and one wall during construction at the Mt. Blue High School gym has been reviewed by two separate structural engineering firms and the architectural firm that designed the $64 million new school.

The experts determined the remaining three walls and a public pathway to other parts of the campus are safe for students and faculty to continue using.

In a letter sent to parents on Tuesday and reprinted on the district’s website (www.mtbluersd.org), architect Lyndon Keck of PDT Architects in Portland explained the three-year project’s ongoing safety practices in light of the accident that left one worker with minor injuries.

“Student and faculty safety is first and foremost in the minds of all members of the construction team because they are working on a site filled with hundreds of students and faculty members,” Keck wrote.

“All three firms feel confident it is safe for students and faulty to continue traveling along the dedicated ‘public’ paths, which have been established for school use,” according to the letter.

Students use the fenced-in paths as they move between the main academic building, the 21 portable classrooms and the nearly completed set of classrooms known as the “E Wing.”

The cause of the collapse of nine of 14 steel joists is being investigated by a team of insurance inspectors along with the State Fire Marshal’s Office.

It will take months to determine what caused the collapse. The district will not be liable for any additional cost, officials said Monday.

“The plan was to remove the east wall of the gym. While they were doing that, something happened to the connections between the roof trusses and the top of the wall. It will be up to the insurance companies to figure that out,” Keck said Monday.

The contractor, Wright-Ryan Construction Inc., had a licensed engineering firm design temporary steel cribbing and shoring to hold up the joists while the east wall was being demolished to be replaced by a stronger firewall, he said.

The original design called for reusing existing joists and adding new steel supports in between. As of Tuesday, no decision had been made on whether to leave the remaining joists or take them all down.

“The original plan was to put one new (joist) in between each old one. But I am pushing for all new ones,” David Leavitt, RSU 9’s director of support services, said.

The 96-foot-long supports are manufactured to order and adding additional structures will add to the delay.

When the collapse occurred, one worker operating a scissors lift was injured. The 23-year-old operator suffered minor injuries when he was thrown clear of the machine, which was covered in debris. Leavitt said he and several others had been in the gym only 20 minutes before the collapse.

The project is already behind schedule, and the incident will delay completion of the gym renovation by two weeks. It was set to open in mid-November.

Parents and community members have contacted Superintendent Michael Cormier with questions about student safety. Keck said that was why he wrote the open letter.

“If I was a parent there, I would want to know,” Keck said.

“Construction is a dangerous business, and crews have to work hard to be careful and safe. The good news here is that we were very, very lucky, and no one was seriously hurt,” he said.

In his letter, Keck highlighted the project’s safety measures:

Every two weeks, administrators meet with the architect, contractor and state agency representatives to review progress and the schedule. The first item on the agenda is usually “school and site safety” to discuss safety concerns and communication between school and contractors.

Construction phases, which are reviewed regularly by public safety officials, the school district and the construction team, define areas safe for use by students, faculty and visitors.

During the project, cranes are not allowed to lift steel, concrete or heavy equipment over occupied areas of the building.

Work planned for times when students are not in the building include foundation and roof replacement work on the academic wing, geothermal well drilling and demolition of the old auditorium, music room and cafeteria.

Wright-Ryan Construction Inc., one of Maine’s largest general contractors with an excellent safety record, has done dozens of projects with fully occupied buildings and has a dedicated safety team work on every project to review safety protocols.

The new school will have a full array of life-safety systems. Fire alarms, telephones, emergency lighting and emergency exits have been reviewed by local and state public safety officials and the State Fire Marshal’s Office, which makes periodic inspections.

PDT Architects has done many large renovations where students and faculty continue to use the building.

The completed project will bring the 40-year-old building up to current structural safety codes with completely new electrical, mechanical, communication and public-safety systems and a fully automatic sprinkler system, which the existing school does not have.

Leavitt said crews working over the weekend removed the debris from the gym, and work is starting on the new concrete-block east wall and to repair any collapse-related damage to the remaining three walls.

Cormier said on Monday he hoped Keck’s letter will address the community’s concerns over safety.

As for the delay in opening the new gym, he said it will mean alternative places for basketball practice need to be found as well as other options for physical education as winter arrives. Classes are already being held in portable classrooms converted to a weight room, a locker room and the cafeteria.

“It is not optimal because, as a school, we are not offering a broad and full-fledged curriculum,” he said.