MINNEAPOLIS - 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."

"We need to find out about their 72-hour history and about their work load. We also want to look into was there any impairment involved here? Was there any distraction involved here? We want to look into all the issues regarding the human part of it," said Hart.

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

"We are asking for the excavators to start removing the debris to make it safe, and we have to do that very carefully because in the removal of the debris we want to be careful not to destroy any evidence that we would need," said Hart.

Wednesday's explosion and partial building collapse killed two longtime school employees and critically injured a third person.