MINNEAPOLIS (AP) — Federal investigators say a father-son team was moving a gas meter at the time of a fatal explosion that leveled part of a Minneapolis school.

The National Transportation Safety Board said Friday that the work was being done Wednesday at Minnehaha Academy as part of gas company CenterPoint Energy's plan to move meters from inside buildings to outside for easier access.

The NTSB is investigating because it has jurisdiction over gas pipelines. NTSB board member Christopher Hart says investigators want to know as much as they can about the contractor, Master Mechanical, and the oversight conducted during the "hazardous operation."

Hart also said Friday that it will take time for investigators to get into the building and examine evidence. He says the building is structurally unsafe, and excavators need to remove debris before investigators can enter.

Wednesday's explosion and partial building collapse killed two longtime school employees.