SEARSMONT, Maine —Federal investigators say firefighters unknowingly used a suppression tactic that likely contributed to the deadly explosion at Robbins Lumber mill fire in Searsmont in May. The report from the National Institute for Occupational Safety and Health (NIOSH) says that neither workers nor responding firefighters understood that the silo involved was designed to limit oxygen or that its contents posed a combustible dust hazard. … A preliminary investigation classified the fire as accidental and identified combustible dust as the cause of the subsequent explosion. The report says the silo was an oxygen-limiting model, which is designed to restrict the amount of air entering the structure. NIOSH said mill employees and firefighters interviewed after the explosion did not know the silo was oxygen-limiting and were unaware that the wood shavings and sawdust inside constituted a combustible dust hazard or that there were different risks with that silo as compared to other designs.