LAWRENCE, Mass. (AP) — Federal investigators are confirming overpressurized natural gas lines were the source of the deadly explosions and fires in communities north of Boston last month.

Thursday's preliminary report from the National Transportation Safety Board says Columbia Gas workers failed to account for the location of critical gas pressure sensors in Lawrence on Sept. 13. It caused high-pressure gas to flood a low-pressure distribution system at excessive levels.

The excessive pressurization led to more than 80 explosions and fires across Lawrence, North Andover and Andover that killed...