Federal investigators are focusing on whether workers moving a gas meter at Minnehaha Academy's upper campus could have caused a Wednesday explosion that ripped a huge gash through the center of the building in south Minneapolis, killing two staff members and injuring nine others.

A team of investigators from the National Transportation Safety Board (NTSB) arrived in the Twin Cities Thursday and took an initial look at the blast site, said NTSB Chairman Christopher Hart.

"We understand that this explosion happened in the course of moving a gas meter; that's what we'll be exploring starting tomorrow," Hart said. He asked for potential witnesses, including those who may have taken photos or videos of the incident on their cellphones, to come forward to help with the investigation.

In the course of the on-site probe, which will start Friday and is expected to last about a week, the team will focus in part on whether workers turned off the gas inside the school rather than at the street before work began, Hart told reporters at a briefing Thursday afternoon. "We want to find out not just what happened, which is relatively easy. We want to find out why it happened," he said.

The natural gas explosion killed receptionist Ruth Berg, 47, and part-time custodian John F. Carlson, 82, who also drove a Metro Transit bus for 32 years until 2005. Fire officials said the two bodies were found near each other, both on the south side in the collapsed portion of the building.

A full-time custodian, Bryan Duffey, was also in the area when the blast occurred and was severely injured.

Duffey, who has been with the school for a year and also is an assistant boys soccer coach, is being treated at Hennepin County Medical Center. Two others remain at HCMC and were in satisfactory condition Thursday.

Hart said he was aware of reports that several survivors reported smelling gas and tried to clear the building moments before the explosion, but that investigators hadn't yet been in contact with survivors or the contractor, Master Mechanical, which city records show had done projects at the school in 2003 and 2004.

The chairman said the NTSB will team up with Minnehaha Academy and local authorities to investigate both the operations and the human aspects of the tragedy.

They'll consider questions ranging from the efficacy of the systems to the training and mind-sets of their operators, including whether they were tired, distracted or impaired.

The cause of the blast will not be determined while investigators are on scene, Hart said, adding that a final report may not be released for a year.

According to city records, Eagan-based Master Mechanical Inc. was issued a permit June 7 for "gas piping and hooking up meter" at the address.

City spokeswoman Sarah McKenzie said that there were no city inspections after the permit was issued.

Standard practice for that type of permit would be for the contractor to call for a "rough-in" inspection that includes pressure testing all new piping, then a final inspection when the job is complete, she said.

Damage to the school was extensive. The blast tore through a center portion of the building, near E. Lake Street in the 3100 block of West River Parkway, where the utility area is located. Window frames were blown out. Classrooms also took the brunt of the blast. Two floors collapsed over a subbasement, which quickly filled with water from the firefighters' hoses used to battle the blaze.

Fred Pritzker, a Twin Cities attorney who has filed dozens of lawsuits following explosions across the country, said the investigation will focus on cause and origin — trying to identify the source of the gas and how it ignited.

"This is not the type of thing that happens in the absence of negligence: Somebody screwed up big time," Pritzker said.

"Whenever you've got an older building, you're always looking to see if these were capped off lines and if they were pressurized," he said of the building, part of which was built in 1912, city records show.

John Purdy, a former boiler inspector, says the building's age may have played a role.

"If you've got a shut-off valve, you can't trust that it'll close on you, if it hasn't been closed in a while," he said.

'Going to get through it'

Along with year-round staff tending to their summertime responsibilities, several athletic teams were practicing at the Upper School when the blast rocked the building and could be heard many blocks away.

"It looks like it could have been an explosion," one firefighter reported over the radio. "We have windows that were blown out on the west side of the building ... we also see now flames and fire and a building collapse as well."

A memo to parents from school President Donna Harris, who was injured in the explosion, said the start date for the damaged Upper School will be pushed back from Aug. 23 to Sept. 5.

Harris said Thursday that administrators are working on finding a facility that best suits the school.

"We are blessed to have received lots of offers," she said.

Crisis counselors will be at the Lower and Middle Schools Friday, Monday and as needed the rest of the week. The Christian school had a combined enrollment of 825 students in 2015-16.

A day after the blast, neighbors and curious residents strolled by the explosion site. In the drizzling rain, they pulled out their phones to take photos of the rubble.

Jim Norman, 70, wearing a Minnehaha Academy cap, stopped in front of the fencing overlooking the soccer field and school behind it. He graduated from the school in 1965 and remembered seeing John Carlson at sporting events.

Losing him and Ruth Berg "is a tragedy," he said.

His family's ties to the school are strong: His wife, brothers and two sisters are all graduates. To see the building reduced to debris is "shocking," he said.

"From the looks of this structure, it's all gonna have to be torn up," he added.

Carolyn Bedingham and her daughter Ellie, of Woodbury, were returning from a trip to Colorado and unable to attend Wednesday evening's vigil. They stopped by Thursday morning with Ellie's friend Laura Shea to place a bouquet at the foot of the fence overlooking the school.

Ellie and Laura have gone to Minnehaha Academy since preschool and will soon start their senior year. They held back tears as they looked at what remained of the halls they knew so well.

"I didn't realize how bad it was until I saw it," Ellie said. "You could've been standing up those stairs when it exploded. When you think about that, it's hard."

All three had appreciative words for Berg and Carlson.

Ellie remembered how Carlson would give them Dilly Bars if they promised to get good grades. Carolyn said he also would come and open up the school for students who had forgotten their homework.

"He loved being there; it was obvious," Carolyn said. "He was part of Minnehaha, in a way; his presence was always there."

Ellie said the loss and damage won't harm the solidarity of Minnehaha.

"It's not the building; we are a community," Ellie said. "It sounds kind of cliché, but it's really true once you experience it. We're gonna get through it, but it's not gonna be easy."

Star Tribune staff writers Matt McKinney, Andy Mannix and Beatrice Dupuy contributed to this report.