Investigators and fire officials in Denison believe they know the partial cause of a grain elevator explosion last week. Denison fire chief Mike McKinnon says grain dust caused the explosion, but they don’t know what ignited the dust. McKinnon says volunteer firefighters continue to work at the scene amid smoking rubble.McKinnon says firefighters could be on the scene for some time to come.The Denison Seed elevator was scheduled to close at the end of the month.