A New Tactic? Dec. 5— Authorities in the United Kingdom are examining a pair of blue socks, which they believe contain key components of an improved explosive device that could be used in future terrorist airline hijackings, intelligence sources told ABCNEWS. Intelligence officials believe the socks were designed to be filled with explosives and tethered and worn around a potential bomber's neck, a source told ABCNEWS. With the explosives concealed in this fashion, a bomber could gain access to an aircraft and assemble the improvised device once on board. The socks were seized during the Nov. 20 arrest of Sajid...