In mid-May, a C.I.A. expert on Al Qaeda briefed Pakistani law enforcement officials on the existence of an elusive operative who was said to be eager to attack Americans, according to a Pakistani intelligence official. Nearly two months later, Pakistani officials traced him to the port city of Karachi and then here. On July 13, they made an arrest in the case, picking up Muhammad Naeem Noor Khan, a 25-year-old computer engineer, when he went to the airport to collect a package from his father.Full Story