 Investigators from the National Transportation Safety Board were in Minneapolis today looking into an apparent natural gas explosion and partial building collapse at Mini Ha Ha Academy that killed two people and injured at least nine including one critically. City fire officials said Wednesday's collapse was caused by a natural gas explosion. Contractors were working in the school at the time and some witnesses said they were warned of a gas leak moments before the blast. Longtime school receptionist Ruth Berg and custodian John Carlson were killed in the building collapse. Three people remained hospitalized Thursday including one in critical condition according to Hennepin County Medical Center. 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 says the agency is investigating because it has jurisdiction over gas pipelines. If you've enjoyed this segment of Lakeland news, please consider making a tax deductible contribution to Lakeland Public Television.