MINNEAPOLIS (AP) — Investigators from the National Transportation Safety Board were sent to a Minneapolis school Thursday to look into an apparent natural gas explosion and partial building collapse that killed two people and injured at least nine, including one critically.The explosion at Minnehaha Academy reduced part of a building on its upper school campus to rubble. City fire officials said Wednesday’s collapse was caused by a natural gas explosion. Contractors were working in the schoolat the time, and some witnesses said they were warned of a gas leak moments before the blast.Some first responders also reported smelling natural gas as they pulled people to safety.The blast occurred in a utility area as students were playing soccer and basketball at school, according to fire and school officials. NTSB spokesman Eric Weiss said the agency is investigating because it has jurisdiction over gas pipelines.Minneapolis Fire Chief John Fruetel said two bodies were pulled from the rubble on Wednesday. While officials have not formally identified them, Minnehaha Academy, said on its Facebook page that the victims were Ruth Berg, a receptionist for 17 years at the school who “welcomed everyone with a smile,” and John Carlson, a part-time janitor known for giving Dilly Bars to students.Carlson, 81, attended the school as a child, sent his own children there, and was like a grandfather figure to students, school officials said. Berg was engaged to be married.Three people remained hospitalized Thursday, including one in critical condition, according to Hennepin County Medical Center. Six other patients who were brought to the hospital after Wednesday’s blast have been released. Dr. Jim Miner, the hospital’s chief of emergency medicine, said victims suffered injuries ranging from head injuries and broken bones to cuts from debris.Minnehaha Academy is a private, Christian school that serves students from pre-kindergarten through 12th grades. The explosion affected only the upper school; the lower and middle school campus is about a mile and a half away. Aerial video footage of the school’s campus showed part of a building was ripped apart, with wood splintered and bricks scattered about. Windows in other areas were blown out and shattered. Three people were rescued from the building’s roof shortly after the explosion and fire, Tyner said.Paul Meskan, who lives across the street, said he was pulling weeds when the blast happened, and he quickly ran to the school. Meskan said he and other people who rushed to help found a man pinned under the rubble.“We just started digging,” Meskan said. He said that after police and firefighters arrived, “we kept digging, and gas, gas was going. Fire was going. And it’s like, ‘We’re not going back until we get this guy out of here.’ And we got him out, and they got him on a stretcher.”The Star Tribune reported that city records show Master Mechanical Inc. was issued a permit on June 7 for “gas piping and hooking up meter” at the school’s address.Master Mechanical said in a statement Thursday that its employees were among the injured. The contractor did not say how many of its employees were hurt.Master Mechanical has twice been cited for workplace violations in recent years, according to the newspaper. Jenny O’Brien, a spokeswoman for the Minnesota Occupational Safety and Health Administration, said in 2010 there was a violation related to protecting an employee from falling. In 2014, the company had paperwork violations.At the time the blast, as many as 10 students were playing basketball inside a gym at Minnehaha Academy but weren’t near the explosion, said Sara Jacobson, the school’s executive director of institutional advancement. Jacobson also was in the building.“There was a very loud explosion, and ceiling tiles and windows and materials rained down on our heads,” she said. “And then soon as it was over, we made our way down a dark hallway to the exit as quickly as we could.”___Associated Press writer Gretchen Ehlke contributed to this report from Milwaukee.