ST. PAUL, Minn. — The fire that destroyed the upscale Seven Corners Gateway hotel project under construction in downtown St. Paul last week was set intentionally, investigators said. The federal Bureau of Alcohol, Tobacco, Firearms and Explosives announced that determination after investigating the ruble, reviewing video footage, and talking to witnesses. The investigation took four days. No human remains were found, the AFTE agency said. Nor has anyone been unaccounted for. The general contractor Doran Corporation’s chief executive, Anne Behrendt, said company was extremely disappointed to learn that the fire was arson. “This destructive act is a considerable setback to a project that was moving the economy forward and supporting construction workers during this time of economic uncertainty,” Behrendt said. Seven Corners was a $69 million project.