An investigation is underway after a natural gas explosion Wednesday morning rocked a school in Minneapolis, injuring several people. One person was confirmed dead and one person was still missing late Wednesday afternoon.

Minnehaha Academy school administrators said the explosion involved a natural gas leak affecting the upper school, which was confirmed by Hennepin County Emergency Management. Gas is supplied by CenterPoint Energy.

According to initial reports, an explosion caused a fire at mid-morning, with a building partially collapsing. At least nine people were taken to the Hennepin County Medical Center, with three in critical condition, four in serious condition and two with “very minor trauma,” according to Jim Miner, chief of emergency services at the hospital.

The upper school campus was minimally staffed for the summer, with year-round employees, girls on campus for summer cross-country practice and some school basketball and soccer players, according to the school.

An investigation is underway, according to CenterPoint Energy.

“We have well-trained, experienced crews on site who are coordinating with local emergency officials to secure the area,” CenterPoint said. “The cause of the incident is not known at this time and we will conduct an investigation to determine the cause. Our focus at this time is making the area safe. Our thoughts are with those who have been impacted.”

Minnesota Gov. Mark Dayton said his office “is in continuous contact with the city of Minneapolis and the Minnesota Department of Public Safety, as emergency personnel respond to this emergency.”

The state, he said, “will provide any and all resources necessary to aid first responders in their efforts to ensure the safety of all those impacted…”