Isso é discutido por Aragorn na partida de Boromir :
It is as Gandalf feared: by some means the traitor Saruman has had news of our journey. It is likely too that he knows of Gandalf's fall. Pursuers from Moria may have escaped the vigilance of Lorien, or they may have avoided that land and come to Isengard by other paths. Orcs travel fast. But Saruman has many ways of learning news. Do you remember the birds?
Não é explicitamente declarado qual, se algum, foi o caso, mas estas são as possibilidades dadas.