WASHINGTON—A Montreal man has emerged as the key figure in a controversy that has dogged Democratic presidential aspirant Wesley Clark during the summer months. Questions have swirled since June when the former NATO commander alleged on national television that he was pressured to link the Sept. 11, 2001, terrorist attacks and Iraqi leader Saddam Hussein in a mystery phone call he received. Clark first implied the call, not long after the attacks, might have come from White House, then later said it came from a Middle Eastern think tank in Canada. He has never identified the caller. As Clark kicked...