Federal investigators say they have preliminary information that Wednesday's explosion at Minnehaha Academy happened in the course of moving a gas meter.
But the National Transportation Safety Board says it will begin its official investigation into the cause on Friday.
NTSB spokesman Christopher Hart told reporters at a Minneapolis news conference Thursday: "Our mission is to understand not only what happened but why, so we can prevent this from happening again."
Hart said investigators will begin their work Friday and expect to be on the scene for five to seven days. But completing a final report on the explosion's cause is a process that typically takes about a year, he said.
The NTSB has not yet been in touch with the contractor that was working on a gas line and meter when the explosion occurred, Hart said, but added that they'll contact Master Mechanical on Friday.
The NTSB is the federal agency that regulates gas pipelines.
One staff member remains critically injured
The explosion and building collapse at the school's Upper Campus killed two staff members. Another remains hospitalized in critical condition.
Minnehaha Academy said in a statement Thursday it's praying for the healing of Bryan Duffey and his family.
Rebuilding fund created
Minnehaha Academy is a private Christian school in Minneapolis that serves children from pre-school through 12th grade.
An alum of the school started a fundraising campaign to pay for rebuilding the Upper Campus, which serves 9th through 12th graders.
The message on the GoFundMe page reads in part:
"The rich Christian tradition of Minnehaha Academy and all that it stands for means so much to so many of us. Let us all now stand shoulder to shoulder with the families of the victims, offer them our fervent prayers and support, and band together to build an even better Minnehaha Academy for current students and generations to come."