While a positive identification of the remains was still pending, the bureau said in a release the deceased was believed to be the missing woman.
The announcement did not say what led agents and investigators from Montana and North Dakota to the site near Williston.
Two Colorado men were arrested last month for allegedly abducting and murdering Arnold, apparently at random while she was jogging near her home in Sydney.
Kate Middleton recycles dress at movie premiere
Benedict Cumberbatch's dramatic reading of R. Kelly lyrics is just what you need