As people crowded into the capital for Barack Obama's inaugural celebration, senior counterterrorism officials huddled in the White House situation room, frantically trying to unravel intelligence about a possible attack on Washington. By Tuesday afternoon, as Obama took the oath of office, the threat of a terror plot by the Somalia-based al-Shabab organization had been debunked, but the flurry of activity underscored growing worries about this Islamic militant group. "I think they are a serious problem, and I don't think that we should be glib and take it lightly," said Theresa Whelan, deputy assistant secretary of Defense for African Affairs....