WASHINGTON (Reuters) - A 14-month investigation by the U.S. Coast Guard and FBI has uncovered nine merchant mariners with possible terrorist links, raising renewed concerns that U.S. ships and ports are vulnerable to attack. Coast Guard spokeswoman Jolie Shifflet said on Thursday that "Operation Drydock," prompted by national security concerns after the Sept. 11, 2001, attacks, had also led to the arrest of about a dozen others whose active arrest warrants for crimes from minor misdemeanors to attempted murder had long gone unnoticed. The Coast Guard said it investigated the records of more than 200,000 people who hold U.S. merchant...