The emergence of a former Guantánamo Bay detainee as the deputy leader of Al Qaeda's Yemeni branch has underscored the potential complications in carrying out the executive order President Obama signed Thursday that the detention center be shut down within a year. The militant, Said Ali al-Shihri, is suspected of involvement in a deadly bombing of the United States Embassy in Yemen's capital, Sana, in September. He was released to Saudi Arabia in 2007 and passed through a Saudi rehabilitation program for former jihadists before resurfacing with Al Qaeda in Yemen. His status was announced in an Internet statement by...