CHARLIEVILLE, Trinidad and Tobago – Five years ago, as 19 al-Qaida operatives in the United States put the finishing touches on what would become the Sept. 11 attacks, a frail, asthmatic computer engineer from South Florida paid a visit to this tiny Muslim enclave where he’d lived as a boy. Adnan Gulshair Muhammad el Shukrijumah, then 25, kept a low profile over the course of the week. He hung out with a small circle of devout older men who were leaders of the local Islamic community. They prayed in mosques, went fishing and enjoyed long walks and leisurely dinners, recalled...