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MINNEAPOLIS — An 81-year-old janitor has been confirmed as the second victim killed after a school building collapsed in an explosion Wednesday, an incident that injured several others.

The blast occurred in a utility area at the Minnehaha Academy as students were playing soccer and basketball. The private Christian school serves students from pre-kindergarten through 12th grade.

Assistant Minneapolis Fire Chief Bryan Tyner said contractors were working on one of the campus' buildings at the time of the blast, which investigators believe was caused by a natural gas explosion.

The explosion killed Ruth Berg, a receptionist for 17 years at the school who "welcomed everyone with a smile," the school said in a statement, and part-time janitor John Carlson, who was found buried in the rubble at 8 p.m. Wednesday night.

Carlson had deep ties to the school. He attended the school as a child, sent his own children there, and was "like a grandfather to students," school officials said. He was known for giving frozen Dilly Bars to students.

John Carlson, killed in the Minnehaha Academy school explosion, Courtesy Minnehaha Academy

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"Minnehaha Academy is deeply saddened by today's events. Tonight we report that we have lost alum and staff member John Carlson. John Carlson was Minnehaha's biggest cheerleader and a long-time presence in the school. He graduated from Minnehaha in 1953, sent his children here, and after retiring from his first career he came back to work at the school. John will be deeply missed. Please keep John's family, Ruth's family, those who were injured, and our school community, in your prayers," a school statement said.

Fire Chief John Fruetel said rescuers were not looking for any other victims.

Minnehaha Academy explosion victim, Ruth Berg. Facebook

Four people remained hospitalized late Wednesday, including one in critical condition, at Hennepin County Medical Center in Minneapolis, according to the hospital. Their names haven't been released.

Dr. Jim Miner, the hospital's chief of emergency medicine, said the victims' injuries ranged from head trauma and broken bones to cuts and abrasions.

Aerial video footage of the school's campus showed part of a building was ripped apart, with wood splintered and bricks scattered about. Windows in other areas were blown out and shattered. Three people were rescued from the building's roof shortly after the explosion and fire, Tyner said.

Emergency workers respond to an explosion at Minnehaha Academy Wednesday, Aug. 2, 2017. Aaron Lavinsky / Star Tribune via AP

Paul Meskan, who lives across the street, said he was pulling weeds when the blast happened, and he quickly ran over to the school. Meskan said he and other people who rushed to help found a man pinned under the rubble.

"We just started digging," Meskan said. He said that after police and firefighters arrived, "we kept digging, and gas, gas was going. Fire was going. And it's like, 'we're not going back until we get this guy out of here.' And we got him out, and they got him on a stretcher."

The Star Tribune reported that city records show Master Mechanical Inc. was issued a permit on June 7 for "gas piping and hooking up meter" at the school's address. Ryan Larsen, a company official, released a statement saying the company was monitoring the situation and referred questions to the Minneapolis Fire Department.

Former Minnehaha Academy employees Elizabeth Van Pilsum, left, and Rick Olson, center, react after an explosion. David Joles / Star Tribune via AP

Larsen wouldn't confirm to The Associated Press that company workers were on site, saying: "We are trying to figure it out."

Master Mechanical has twice been cited for workplace violations in recent years, according to the newspaper. Jenny O'Brien, a spokeswoman for the Minnesota Occupational Safety and Health Administration, said in 2010 there was a violation related to protecting an employee from falling. In 2014, the company had paperwork violations.

At the time of Wednesday's blast, as many as 10 students were playing basketball inside a gym at Minnehaha Academy but weren't near the explosion, said Sara Jacobson, the school's executive director of institutional advancement. Jacobson was in the building during the explosion.

"There was a very loud explosion, and ceiling tiles and windows and materials rained down on our heads," she said. "And then soon as it was over, we made our way down a dark hallway to the exit as quickly as we could."

Gov. Mark Dayton released a statement saying his office was in contact with city officials and the state "will provide any and all resources necessary" to help first responders and ensure everyone is safe.