Minnehaha Academy senior Chimali Day was in the school counselor's office, her parents by her side, when the sudden warning came.

"Get out!" a staffer cried, breaking the Wednesday morning calm on the upper school campus. "There's a gas leak."

Day leapt for the door just as the center of the south Minneapolis private school building exploded, the blast knocking her to the floor. Her mother jumped on top of her, instinctively sheltering her child with her body. Then they scrambled through the back door of the counseling office while Day's father went looking for her classmates.

The teenager was among the students and staff who escaped the natural gas explosion that killed receptionist Ruth Berg, 47, and janitor John F. Carlson, 82, and injured nine other people, one critically. Fire officials said the two bodies were found near each other, both on the south side of the rubble.

Staff and parents at the Christian school along the Mississippi River gathered together to grieve Wednesday night, even as they gave thanks that fall classes were not yet in session when the blast occurred.

"Tonight is an example of the kind of caring community we are," Minnehaha Academy President Donna M. Harris, who was injured in the explosion, said during the prayer service at the academy's lower and middle campus. Seven hundred people packed the campus chapel for the emotional event, which featured hymn-singing, words of comfort, and many tears.

"We're going to get through it," Harris said. "We trust God. He is going to do phenomenal work."

The explosion rocked and set ablaze the upper school at 10:23 a.m., causing a partial collapse. After the fire was extinguished, an intensive operation began to find the missing. The first body, later identified as that of Berg, was found about 2 p.m., according to Fire Chief John Fruetel. The second, that of Carlson, was found at dusk.

As of dawn Thursday, assistant boys soccer coach Bryan Duffey remained hospitalized in critical condition and two were in satisfactory condition at Hennepin County Medical Center. The six others, including Harris, were treated and released.

Dr. Jim Miner, the hospital's chief of emergency services, described the injuries as fractures, cuts and head wounds. He said no one was treated for burns.

"It could have been a lot worse," he told reporters outside the hospital. "We were relieved; we were prepared for a lot more injuries." Still, he added, "It was a terrible tragedy."

After staff from the Hennepin County medical examiner's office removed Carlson's body from the blast site late Wednesday, fire investigators declared an end to their recovery operation.

The next phase — the investigation into the blast's cause — begins Thursday. A rapid-response team from the National Transportation Safety Board was heading to the Twin Cities to investigate the leak. The agency said it planned to brief the news media sometime Thursday afternoon.

A desperate effort

In the first moments after the blast, the peril of leaking natural gas proved daunting.

Police officer Dean Milner said he and others saw one person under the rubble and were told there was another they could not see. He and the other officers broke apart what remained of a demolished wall and freed the first person. But because of flames, heat and the lack of heavy equipment, they couldn't reach the other person. And they smelled gas.

"We're getting told as we're digging them out that there is gas leaking into the basement, that there is a big drum in the basement and the flames were getting close to us as we almost had the debris off of him to get him out," said officer Vicki Karnik. "I think the hardest part was not being able to help any further."

The building, which is near E. Lake Street in the 3100 block of West River Parkway, collapsed mostly in the center portion, where the utility area is located and where a chimney was damaged but still standing. Window frames were popped out of walls. Classrooms also took the brunt of the blast.

Two floors collapsed over a sub-basement that soon was filled with water from the hoses used to battle the fire, Fruetel said.

Agents with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives were on scene interviewing witnesses, including one whose head was bandaged. Minneapolis homicide detectives took witnesses one by one into a Metro Transit bus for interviews.

The natural gas supply from CenterPoint Energy was cut off, and the utility sent crew members to the site to help secure the area and investigate the leak, said CenterPoint spokeswoman Becca Virden.

Fruetel said there had been reports of a gas leak before the collapse.

"There was some warning to get out of the building. I do know that," he said.

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

Ryan Larsen, a financial officer with the company, said, "We've got people on the site there. They are figuring it out."

In a statement later, Master Mechanical said, "We are forever grateful to those first responders and bystanders who came to the aid of all of the injured, including our employees."

Workplace inspections of Master Mechanical have led to two citations for violations in recent years, neither of which carried major penalties, according to Jenny O'Brien, a spokeswoman for Minnesota OSHA. The company was initially fined $1,100 in 2010 for a violation related to an employee's fall on a Bloomington site.

In 2014, it was initially fined $600 for two paperwork violations at a Maple Grove site, involving an employee right-to-know and a hazard communication, OSHA records show. Those fines were later reduced, to $770 (2010) and $415 (2014).

A blast, and terror

Founded in 1913, Minnehaha Academy is a Christian school. It had 825 students enrolled during the 2015-16 school year.

Among those on the upper school campus at the time of the blast were year-round staff, girls participating in a summer cross-country practice, as well as school basketball and soccer players. Classes were scheduled to begin Aug. 23.

Kylee Kassebaum, a sophomore cross-country runner, and her teammates had left the building for lunch about 30 seconds before the blast.

"All the windows just kind of burst out, and there was a huge explosion that was so loud it kind of shook your insides," she said. "As soon as we saw it we got in the car, and someone drove away because we didn't know how safe the situation was."

School administration official Jacob Swanson said he and others were in a meeting when, "I heard a loud explosion. My ears popped."

Swanson said that he and the others "just saw the middle of the building gone."

A girls' basketball team was also practicing in the gym when the blast occurred. A coach described it as sounding like "a large door slamming."

Tom Haubrich graduated from the academy in May and rushed there after seeing news reports.

"That right there is where everybody would've been during the school year," he said, pointing to the wreckage. "That's the stairwell that connects the entire school to everything."

Lunchrooms, libraries, classrooms, "everything connects to right there," he said. "If it had been during the school year, hundreds would've been hurt."

Dozens of neighbors lined 46th Avenue S. as they watched units douse the rubble from across the school's soccer field and behind yellow caution tape.

Anne Dussol, who lives a block away from the school, said she heard the explosion, and "there were flames shooting from the parking lot. We thought at first it was a car on fire until we realized the building was gone."

Kids were playing soccer on the athletic field at what she believed was part of a soccer camp, she said. After the explosion, they ran from the school and onto the street.

Deb Shold said she was in the basement about a block away, when the explosion rattled her home. Two minutes later she grabbed her watch, which read 10:25 a.m. "I heard a very loud explosion, and the lights dimmed but came right back on," she said.

'We are going to rally'

All day, condolences poured onto social media for Berg, who was engaged to be married, according to future brother-in-law Jeff Burrington, and Carlson, a Minnehaha alumnus who returned in 2003 to work as a janitor and was well-loved by students and faculty alike.

At Wednesday night's prayer vigil to honor them, tears flowed freely.

"No one thought this morning this was going to happen," said Mark Stromberg, superintendent of the northwest conference of the Evangelical Covenant Church, acknowledging the mourners' shock.

After the service, students from the upper campus gathered at the back of the chapel, where Principal Jason Wenschlag told them that although he doesn't have answers to why the explosion happened, "Everything is going to be OK."

"We are going to rally," he said.

Star Tribune staff writers Beatrice Dupuy, Andy Mannix, James Eli Shiffer and Karen Zamora contributed to this report.