The Charlotte Fire Department is working to determine the cause of a structure fire in north Charlotte Thursday morning. The incident occurred just before 4 a.m. on Keswick Avenue right off North Tryon Street. Fire crews said the structure was a two-story house that was under construction. “A fire can spread quickly in an open building before it gets dried in with drywall, so a small amount of fire can spread very quickly with exposed wood,” Battalion Chief Blake Redden said.