For 10 days this summer, Umar Farouk Abdulmutallab took language classes in this ancient city on the Arabian Peninsula. He lived in student housing, appearing to his fellow students to be devout, friendly and generally content.

Then, he was gone.

New details in the case of Mr. Abdulmutallab, charged with attempting to bring down Detroit-bound Northwest Airlines Flight 253, have emerged suggesting that it was around this time that the young man met with the radical U.S.-born cleric Anwar al-Awlaki, according to a person familiar with intelligence shared among Arab states and a U.S. official. The person familiar with Arab intelligence says Mr. Abdulmutallab met with a mysterious Saudi operative of Al Qaeda.

A few months later, on Christmas Day, he boarded the plane for Detroit, with 76 grams of explosives allegedly sewn into his underwear.

Investigators in the U.S. and Yemen believe the meetings marked a critical turning point in Mr. Abdulmutallab's gradual transformation from pious Muslim to alleged terrorist How and when his relationships were initially forged with Al Qaeda and Mr. Awlaki, who has surfaced in multiple terror probes, is at the heart of the global scramble to trace Mr. Abdulmutallab's "radicalization"—and to determine how authorities could have missed the warning signs.

Through most of his life, the Nigeria-born Mr. Abdulmutallab came off as a religious and inward young man, so opaque as to be virtually unknowable. He was intense and serious about Islam, but in a way that acquaintances judged to be within the mainstream.

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