A British Taliban fighter in custody in Guantánamo Bay provided U.S. investigators a crucial link between a group of Seattle militant Muslims and members of a radical mosque in London, federal sources say. The prisoner, Feroz Abbassi, told CIA interrogators earlier this year that he had traveled to Afghanistan from London in 2000 with an American — a Muslim convert now suspected of being a key figure in the Seattle group suspected of supporting the al-Qaida terrorist network. Abbassi said he had met the man at the North London Central Mosque, a major recruiting ground for Islamic terrorists. That mosque...