The U.S. government finally acknowledged Friday it cannot determine which refiners and smelters around the world are financially fueling violence in the war-torn Congo region.The Commerce Department published a list of more than 400 sites from Australia to Brazil and Canada, but said it “does not have the ability to distinguish” which are being used to fund militia groups...The inconclusive report underscores the challenges faced by hundreds of U.S. public companies that also had to comply with the rule and file reports on their efforts to discover conflict minerals in their supply chains by June 2. Companies including Intel...and Apple said they spent years and millions of dollars investigating their supply chains to figure out which components might contain gold, tin, tungsten and tantalum from mining operations blamed for funding armed militia groups in the Democratic Republic of the Congo. The vast majority said they also couldn’t be certain, but a dozen companies, including Google Inc., J. Crew Group Inc. and Deere & Co., acknowledged their suppliers may have obtained metals from such mines.