The Amber Alert was issued in Washington and Oregon on Wednesday afternoon.
Hannah Anderson and her brother, Ethan, were last seen Aug. 3, and are believed to have been abducted by James Lee DiMaggio.
DiMaggio is suspected of killing the children's mother, 42-year-old Christina Anderson.
Her body was found in a burned house along with the remains of a child. Authorities said it could take days for DNA testing to determine if the child is Ethan.