Federal accident investigators were on-site Saturday at the Sunoco Logistics terminal in Nederland to discover how a welding contractor might have touched off a flash fire late Friday on a crude oil pipeline connection under construction, a Sunoco Logistics spokesman said. "There was welding, a crude oil line and fumes," said Jeff Shields, Sunoco spokesman, also on-site along with representatives of the work contractor, Houston-based L-Con Inc. Seven contract workers were injured in the flash fire, four of them critically. Three were air-lifted to burn specialist hospitals in Houston and Galveston the rest went to local hospitals by ambulance, Shields said.