Emergency crews on Friday were preparing to labor through the weekend to clean up the largest U.S. crude oil spill in nearly a decade, with workers descending on this farming community from as far away as Mississippi. A heavy odor of oil hung in the air, according to a Reuters witness, as tractor trailers ferried generators, lighting and ground mats to a muddy site. Federal investigators were at the scene trying to help determine what caused a leak of some 14,000 barrels of oil from Western Canada, an official said.