LONDON -- About the time Umar Farouk Abdulmutallab left University College London here in June 2008, the U.S. issued a multi-year, multi-visit visa to the Nigerian national, who hails from a privileged background.

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Police enter a property in a mansion block in London on Saturday under investigation in connection with a failed attack on a U.S. airplane.

A senior U.S. official says there was no "derogatory information" concerning the 23-year-old, nor "any information suggesting his radicalization."

What happened between mid-2008 and Christmas Day – when Mr. Abdulmutallab was implicated in the attempted terrorist bombing of a U.S. jetliner – remains a mystery.

"Our information suggests this happened later," a U.S....