You never hear about people like Sheila Horan until something truly terrible happens. But after more than two decades in the FBI's secretive national-security division, she knows her way around terrorism. And now she has been thrust into one of the most difficult manhunts in her career, as on-the-scene boss of the investigation in East Africa that the U.S. hopes will one day nail down the names and addresses of the terrorists who ruthlessly massacred 257 innocents and wounded more than 5,000 in the twin bombings of the U.S. embassies.
Agent Horan found herself in Nairobi last week presiding over a...