The fate of three of nine foreigners abducted in Yemen last week is known — their bodies were found, shot execution style. The whereabouts of the other six — including three children under the age of 6 — remain a mystery. But terrorism experts say their abductors and killers are almost certainly not a mystery. They say the crimes bear the mark of Al Qaeda, and they fear they are the handiwork of the international terror organization's No. 2 man in the Arabian Peninsula: Said Ali al-Shihri, an Islamic extremist who once was in American custody — but who was...