(CNN)Two people were unaccounted for and nine were hospitalized Wednesday after a natural gas explosion shook Minnehaha Academy and collapsed part of a building, officials in Minneapolis said.
One witness said two men yelled "gas!" shortly before the blast.
Fire crews were searching for possible victims in the debris at the private school, officials said. Minnehaha Academy said all of its summer-program students and staff were accounted for and safe. An individual believed to be missing was later found outside the area, officials said.
Three of the hospitalized were in critical condition and four were in serious condition, said Dr. James Miner of Hennepin County Medical Center. All are adults. The patients had fractures and head injuries, but no burns, he told reporters.
The Minneapolis Fire Department initially said one person was killed. But it later tweeted the death was not confirmed.
Authorities said early indications are the blast may have been caused by a ruptured gas line. The circumstances were not immediately known.
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