PORTLAND, Maine (AP) — Firefighters who responded to a deadly fire and explosion at a Maine lumber mill used tactics that inadvertently helped cause the blast, according to a report from federal investigators. The May 15 fire and explosion killed three people, including two firefighters, and originated at the base of a silo, Maine authorities have said. The silo was an oxygen-limiting structure, according to a report released by National Institute for Occupational Safety and Health this month. At the scene, firefighters flowed water into the silo, though the manufacturer’s emergency procedures warn against that, the report said. The report states in its recommendations that fire departments should make sure firefighters choose appropriate tactics for silo fires and never direct water or foam into oxygen-limiting silos.