(Reuters) - Authorities in Minneapolis were investigating on Thursday whether contractors working at a Christian private school caused a gas explosion that leveled a building, killing two staff members and injuring nine other people.

“Various agencies will be on site this morning to pinpoint exactly how this happened,” the fire department’s assistant chief, Bryan Tyner, said on Thursday. “That investigation will include local, state and federal agencies.”

Minneapolis fire department officials told reporters on Wednesday there were early indications that a gas line was ruptured by contractors working on the campus.

The National Transportation Safety Board (NTSB), which investigates accidents involving pipelines, said on Thursday it had sent an emergency response team to the site and that its officials were expected to brief reporters later in the day.

The federal Bureau of Alcohol, Tobacco, Firearms and Explosives has also sent agents and trained canines to join the investigation.

School receptionist Ruth Berg and custodian John Carlson were killed by the blast that tore through the Upper School of the Minnehaha Academy at about 10 a.m. local time on Wednesday, officials said.

“Please keep John’s family, Ruth’s family, those who were injured, and our school community, in your prayers,” the school said on Facebook.

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The school described Carlson, 82, as its “biggest cheerleader.” He had graduated from Minnehaha in 1953. Berg, 47, was engaged to be married, local media reported.

One of the injured was in critical condition at a local hospital. He was named as Bryan Duffey, an assistant soccer coach at the school. One other person was listed as satisfactory, while seven were treated and released, the Hennepin County Medical Center said on Thursday.

“We pray for his healing and for the wisdom of the doctors and nurses caring for him at this time,” the school said in a statement on Thursday, referring to Duffey.

Minnehaha Academy was founded in 1913 and teaches more than 800 students from pre-kindergarten through 12th grade on two campuses, according to its website.

School was not in session on Wednesday due to the summer break. City fire officials said the situation would have been far worse if pupils were in class.

Hundreds of people crowded into a chapel for a prayer service at the academy on Wednesday night.

“We’re going to get through it,” said Minnehaha Academy President Donna Harris, who was injured in the explosion. “We trust God. He is going to do phenomenal work.”