In late August, during the weekend between the Democratic and Republican conventions, America’s military and intelligence agencies intercepted a series of messages from Al Qaeda’s leadership to intermediate members of the organization asking local cells to be prepared for imminent instructions.

An official familiar with the new intelligence said the message was picked up in multiple settings, from couriers to encrypted electronic communications to other means. “These are generic orders,” the source said — a distinction from the more specific intelligence about the location, time, and method of an attack. “It was, ‘Be on notice. We may call upon you soon.’ It was sent out on many channels.”