When the blast ripped through a Minneapolis high school Wednesday morning, teenage girls in basketball practice felt the building shake and saw the ceilings cave in. The few students on campus were knocked off their feet as windows shattered and multiple floors crumbled.

The natural gas explosion ravaged the upper campus of Minnehaha Academy, piercing a giant hole in the structure, sparking a blaze and leaving mounds of debris where a stairwell, utility area and classrooms previously stood. Three people were rescued from the school’s roof, the Associated Press reported.

After firefighters doused out the flames, they began to scour the rubble for missing school employees. By the afternoon, the body of Ruth Berg, a 47-year-old receptionist who was engaged to be married, was pulled from the debris. As night fell, fire officials said they found the body of the other missing person: John Carlson, a beloved janitor in his early 80s known for handing out ice cream bars to students.

The blast also left nine others injured, including one who remains hospitalized in critical condition. Three others remained in the hospital Wednesday night in “satisfactory condition,” according to Hennepin County Medical Center. Five others were treated and later released.

Although some students were in the building at the time of the blast, classes were not in session due to the summer break. The tragedy could have been “dramatically worse,” Minneapolis Fire Chief John Fruetel said in a Wednesday night news conference.

“You can only imagine what that would’ve been like,” Fruetel said. “We were pretty lucky.”

All of the people hospitalized suffered from blast-related injuries such as broken bones, cuts from flying debris and injuries to arms, legs, ribs and vertebrae, Jim Miner, chief of emergency medicine at Hennepin County Medical Center, told reporters outside the hospital. None of the victims were treated for burns, and most of them were conscious upon arrival at the hospital.

When Miner first heard the explosion had occurred at a school, “my thoughts were that it was really going to be bad.”

“This was a tough day, people died and a lot of people were hurt,” Miner said, “but I’m glad it was not worse.”

Berg and Carlson were recovered from the same general area, near an exterior door, Fruetel said, but one of the bodies was found deeper in the rubble than the other.

At the time of the blast, contractors were working on one of the campus buildings, Assistant Minneapolis Fire Chief Bryan Tyner told the Associated Press. Those inside the building learned there was a natural gas leak and began evacuating, the Star Tribune reported. Fire officials confirmed reports of the gas leak, but said the cause of the explosion was still under investigation.

Minnehaha Academy is a private Christian school for students in prekindergarten through grade 12. The explosion took place at the academy’s upper school, which enrolls about 825 students grades 9 through 12. At the time of the blast, year round staff members were in the building along with student basketball players. Other students were outside at cross-country practice and playing soccer.

Kylee Kassebaum, a sophomore cross-country runner, had left the building to get lunch just seconds before the blast. She told a Star Tribune reporter she saw the windows “just burst out.”

The explosion, she said, “kind of shook your insides.”

Chimali Day, a senior at Minnehaha Academy, told the Star Tribune she was in her counselor’s office with her parents when she heard an employee yell, “Get out, there’s a gas leak!”

“As soon as I was getting out it just happened,” Day said. The blast knocked her to the floor.

“My mom jumped on top of me,” she said.

Residents who live near the school could hear the explosion from blocks away. One nearby neighbor, Anne Dussol, told the Star Tribune “there were flames shooting from the parking lot.”

Minnesota Gov. Mark Dayton said in a statement he would provide response teams with all necessary support and resources. In a news conference, Minneapolis Mayor Betsy Hodges said it was “a day of extraordinary heartbreak in Minneapolis.”

“The school is at the heart of a neighborhood and at the heart of a community,” Hodges said.

Hundreds of people packed into a school chapel at the academy’s lower and middle campus for a prayer service Wednesday night. In honor of Carlson, an alumnus handed out Dilly Bars, the Dairy Queen ice cream treats the custodian liked to give to students.

“We’re going to get through it,” Minnehaha Academy President Donna M. Harris said at the vigil. She was in the school at the time of the explosion.

Berg, the receptionist who died in the blast, worked for Minnehaha Academy for 17 years, the school said in a Facebook post.

“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 message said.

A woman who identified herself as Berg’s sister, and who lives in Idaho, wrote on Facebook that she is “trying to hold it together.” She said it is “terribly hard not being with my family at this difficult time.”

“Ruth often spoke of her friends and you meant the world to her,” Berg’s sister wrote, adding that she also spoke of her job and co-workers a lot. “Believe me when I say she loved you to bits and loved her work.”

Keith Kostman, 54, worked at Minnehaha Academy for eight years in the information technology department. In an interview with The Washington Post he said Berg was “always very positive, always smiling.”

“We always had this thing when I would walk through reception,” Kostman said. “We would always high-five each other.”

He said the location where she worked was right above the site of the explosion, “right there at the epicenter.”

Seeing the images of the obliterated building, Kostman said: “It was crazy, the whole thing was gone. It was pretty incredible.”

He also vividly remembered Carlson, the custodian, whom he fondly described an unofficial school historian.

“He knew everything about the school,” Kostman said. “He could tell you stories about the buildings, about people. He could tell you anything about the history of Minnehaha Academy.”

Carlson graduated from the school in 1953, sent his children there and came out of retirement to work at the school as a janitor 14 years ago, Minnehaha Academy wrote in a Facebook post, calling him “Minnehaha’s biggest cheerleader.”

On social media, students shared memories with the kind custodian. Many of these moments involved his trademark Dilly Bars.

“He is the sweetest man in the world,” one message read. “He once saw me crying and gave me a dilly bar and told me that tomorrow is a new day.”

“He even walked me all the way over to Dairy Queen one day to buy me a dilly bar because he had run out of them,” another student wrote on Facebook.

Carlson’s wife, Barbara Carlson asked for privacy when reached by phone late Wednesday just before authorities announced they had recovered her husband’s body.

“I don’t know yet what’s going on,” she told The Post. “I’m trying to keep the line open, waiting for family.”

Later in the evening, a woman who identified herself as Carlson’s daughter posted a note on Facebook that said she was on the school property when she saw an ambulance leave the site with what she believed was her father’s body.

“It was a long drive home,” she wrote.