Two Minnehaha Academy staff members are dead after a natural gas explosion and partial building collapse Wednesday morning at the school's upper campus in Minneapolis.

Late Wednesday afternoon, Minnehaha Academy identified one of the victims as receptionist Ruth Berg.

Ruth Berg, a 17-year employee of Minnehaha Academy in Minneapolis Facebook

Then, around 8 p.m. Wednesday evening, a second body was pulled from the rubble, according to Minneapolis Fire Chief John Fruetel.

Minnehaha Academy confirmed in a statement that the second victim was John Carlson, an 82-year-old custodian.

Contractors were doing work on the building at the time of the explosion, which investigators determined was caused by a gas explosion, according to Assistant Fire Chief Bryan Tyner.

A utility-boiler room was the apparent epicenter of the explosion. "Gas is dangerous," chief John Fruetel said. "You can see the destructive force."

Nine people were taken from the school to Hennepin County Medical Center, including three who were in critical condition and four in serious condition. Late Wednesday afternoon, HCMC said four patients remained: One, Bryan Duffey, was in critical condition, and three were listed as satisfactory. Five others had been discharged.

After the explosion, three people were rescued off the roof of one of the school buildings.

Neighbors embrace along 31st Street in Minneapolis, just outside of Minnehaha Academy's Upper Campus. Evan Frost | MPR News

John Carlson, who was killed in the explosion, graduated from Minnehaha Academy in 1953. He went back to work there 14 years ago, his wife Barbara Carlson said.

The school said Carlson was "Minnehaha's biggest cheerleader and a long-time presence in the school." He sent his own children there and returned to work on staff after retiring from his first career.

Barbara said her husband wasn't scheduled to be at work until 3 p.m. on Wednesday, but he clocked in early because she had a doctor's appointment later in the day.

Grace Kirkpatrick, a Minnehaha student and co-captain of the cross country team, said Carlson went out of his way to engage students.

"He would hand out Dilly Bars to students after school," she said. "He'd always want to start up a conversation with you. He was super-helpful and would go out of his way to make your life a little bit easier."

Minnehaha Academy said Ruth Berg, who was also killed, had worked at the school for 17 years.

"As our receptionist, she welcomed everyone with a smile and was always willing to go the extra mile to help our students, families, and staff," the school's statement read. "She will be greatly missed.

Kirkpatrick called Berg "the sweetest person at school."

She also said that the campus room, which might have been destroyed in the blast, is a gathering area where a lot of students eat lunch, do homework, and just hang around with their friends.

Minnehaha Academy Upper School William Lager | MPR News

Minnehaha Academy is on summer break, but many students and staff were at the campus for basketball practice, soccer and cross country activities. The school said all its summer program students and staff were accounted for and safe.

"You can only imagine what that would've been like," if school had been in session during an explosion, chief Fruetel said.

The pre-K through 12th grade Christian school has multiple buildings. In a post on its Facebook page, the school said the explosion only affected the "upper school," and staff inside the building reported hearing a loud noise and feeling the building shake. The academy's lower school was unaffected.

Minneapolis Fire Department Chief John Fruetel speaks at a news conference Wednesday afternoon, several hours after the explosion. Caroline Yang for MPR News

Joel Maart said he was in the school, talking to a colleague, when it exploded. Another co-worker ran by saying, "get out, get out, there's a gas leak!"

"As soon as I started to take a step or two toward my door, then it just exploded," he said. "It felt like somebody pushed you down, like you got pushed down to the ground pretty good, and dust and stuff flying everywhere."

In order for a gas leak to turn into an explosion, there needs to be a "perfect storm" of conditions, said Mark McDonald, president of NatGas Consulting. Blasts occur only in confined spaces where the air has gas concentrations of between five and 15 percent, he said. Then there needs to be an ignition source to spark the gas.

Investigators have yet to pinpoint what triggered Wednesday's blast.

Gov. Mark Dayton's office released a statement saying state officials were in close contact with local emergency responders and will provide help needed.

Operations at the scene ended before 10 p.m. Wednesday night, with investigations set to continue Thursday morning.

MPR News reporters Tim Nelson, Riham Feshir, Jon Collins, Peter Cox, Laura Yuen, Brandt Williams and Matt Sepic contributed to this report. Editing by Nancy Yang, Paul Tosto and Cody Nelson.