Abu Ubaida al Masri's Kevo

If Al Qaeda strikes the West in the coming months, it's likely the mastermind will be a stocky Egyptian explosives expert with two missing fingers.

His alias is Abu Ubaida al Masri. Hardly anyone has heard of him outside a select circle of anti-terrorism officials and Islamic militants. But as chief of external operations for Al Qaeda, investigators say, he has one of the most dangerous -- and endangered -- jobs in international terrorism.

He has overseen the major plots that the network needs to stay viable, investigators say: the London transportation bombings in 2005, a foiled transatlantic "spectacular" aimed at U.S.-bound planes in 2006, and an aborted plot in this serene Scandinavian capital last fall.

But pursuers have captured or killed his predecessors and have been gunning for him. He prowls Pakistani badlands one step ahead of satellites and security forces.

Although periodic reports of his death have proved false, rumors resurfaced after recent American airstrikes. Asked whether Masri is alive, a Western anti-terrorism official said, "It's a question mark."

Masri himself can be described that way. Authorities know only bits and pieces of his biography. They know his face, having identified an unreleased photo, but not his real name.

"He is considered capable and dangerous," said a British official, who like others in this report declined to be identified. "He is not at the very top of Al Qaeda, but has been part of the core circle for a long time. He is someone who has emerged and grabbed our attention as others were caught or eliminated in the last couple of years. Perhaps he rose faster than he would have otherwise."

The Times has charted Masri's rise in interviews with anti-terrorism officials and experts from Europe, the United States and the Middle East, and a review of case files and academic and intelligence reports. The stories of man and network intertwine, revealing the dangers and vulnerabilities of both.

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