(CNN) A team from the National Transportation Safety Board arrived in Minneapolis on Thursday to investigate a natural gas explosion that killed two people and injured nine at a city school.

NTSB spokesman Christopher Hart told reporters that the agency, through its Office of Railroad, Pipeline and Hazardous Materials Investigations , has jurisdiction due to reports that Master Mechanical, a contracting company, may have been moving a gas meter before the explosion happened.

The blast at Minnehaha Academy on Wednesday was caused by contractors doing work on the building, Minneapolis Assistant Fire Chief Bryan Tyner said.

Two floors in the center section of the building collapsed over a sub-basement, Minneapolis Fire Chief John Fruetel said, and there was heavy damage all through the school. The structural integrity of the building is still in question.

The school identified the dead as receptionist Ruth Berg and staff member John Carlson.

Berg worked for the academy for 17 years, the school said in a statement . "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 statement read. "She will be greatly missed. Please keep Ruth's family, and our school community, in your prayers."

Ruth Berg

Carlson was an alumnus and staff member who was described by the academy as "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," the statement read.

The nine injured people were taken to the Hennepin County Medical Center. On Thursday, the hospital said six had been released. One person was in critical condition and two were in satisfactory condition, the hospital said.

Dr. James Miner of Hennepin County Medical Center told reporters the patients had fractures and head injuries, but no burns.

Fruetel said all construction workers who had been renovating the building were accounted for, and Minnehaha Academy said all its summer program students and staff were also accounted for.

Explosion caused by contractor work

Becca Virden, a spokeswoman for CenterPoint Energy, said crews were called to the area of a school boiler room to assist first responders and make the area safe.

The Christian school said there was a gas leak and explosion at its multistory Upper School, which houses grades 9-12. The academy has about 825 students in grades pre-K through 12, housed on two campuses. The 2017-18 school year is set to begin August 23.

"I would imagine it would have been a lot more significant if it was (during) school time," Tyner said. "It is a good thing that it's the summertime and probably limited the amount of people that were in the building."

The NTSB has jurisdiction over all pipeline incidents, but will typically only investigate them if they involve fatalities and building damage. Hart said the agency will investigate on scene for at least five days to collect evidence and talk to the parties involved. The agency is also encouraging witnesses to come forward.

Witnesses detail harrowing scene

First responders said it looked like a "great big hole" in the building where the collapse occurred, with flames shooting from one side of the building. Officer Dean Milner said two civilians on scene helped rescue one man who was buried beneath the rubble.

Milner said officers were battling through thick flames and the smell of gasoline to rescue the unidentified adult male. Officers also warned people nearby to get away in case of a second explosion.

Officer Vicki Karnik, one of the first responding officers on the scene, said the explosion reminded her of the 2007 Minneapolis bridge collapse that killed 13 people and injured 145.

"I was kind of having a flashback, especially with it being 10 years ago (yesterday)," she said.

Video broadcast by WCCO showed smoke and flames rising from the debris.

Jack Mahler was warming up for a soccer practice nearby when he heard two sprinting men yelling "Gas!" and "Get out!" Shortly after, there was a huge explosion that knocked him off his feet, he told WCCO, "and then it was just kinda chaos from there."

Three people on the roof of the school needed assistance getting down, officials said.

John Barron, who lives across the street from the academy, told the station the blast shook his windows and startled his dogs.

The center section of the building collapsed, authorities said.

"I saw that where the building used to be one continuous building, (it) now had a gap," he said. "I could see sunlight all the way through to the other side."

Gov. Mark Dayton pledged resources for first responders. "I thank the many firefighters, paramedics, and law enforcement officers who rushed to the scene this morning, and who are working still to ensure the safety of our children, adults, friends, and neighbors," he said in a statement.

Friends, loved ones mourn victims in vigil

Hundreds of friends, neighbors, co-workers and community members gathered for a vigil Wednesday night for the victims of the explosion. The vigil was held at the Minnehaha Academy Lower & Middle School, which quickly filled.

Jeff Wagner, a WCCO reporter, tweeted that many of the mourners donned Minnehaha Academy clothes and colors as they bowed their heads in prayer and joined hands in song.

The vigil lasted more than an hour and included speakers who emphasized the importance of retaining a religious faith in times of tragedy and praying for those who have been affected by the incident.

"Let this be a reminder to each and every one of us, no one got up this morning thinking that this was going to happen," one of the speakers at the vigil said. "People were just going to work, people were just serving in their position."