Why do Sohrab and Amir travel to Islamabad in The Kite Runner?

Following Amir's dangerous trip to Afghanistan to
locate Sohrab, he discovers that his nephew is being held hostage by the Taliban. Amir
is surprised to find that Sohrab is being held in Baba's former Kabul home--by none
other than Assef, the same boy who had sodomized Sohrab's father many years before.
With help from Sohrab, Amir manages to escape Assef's near-deadly beating, and the two
head for the Afghan border with their driver, Farid. Amir awakes in a Peshawar hospital,
and after a short stay, he and Sohrab say farewell to Farid and leave for the relative
safety of Islamabad. Amir knows that the Taliban will attempt to find him and Sohrab;
they worry of possible retaliation for Assef's blinding and another kidnapping of
Sohrab. They will be safer in Islamabad, and Amir hopes to find the American couple who
Rahim Khan has told him will take care of Sohrab. When Amir finds that Rahim has made up
the story about the fictitious Caldwells, Amir realizes that he must return to America
with Sohrab. At the American Embassy in Islamabad, he can take the proper legal steps to
adopt Sohrab and return to California.