While the U.S. government is giving out warnings of possible terrorist attacks, they are not revealing where the information is coming from. This is standard practice, as any sources inside, or close to, al Qaeda would not last long if it were revealed they were passing on information. But most of the information obtained is probably not coming from sources in al Qaeda, but from another intelligence technique called "traffic analysis." When all you can do is detect enemy messages, without being able to read them (because it is coded, or not a strong enough signal to read), you can learn a lot from how many messages there are and where they are coming from. U.S. intelligence agencies have compiled a lot of data on how al Qaeda communicates via phone, the Internet, couriers and mail. Over the last year, deals have been made with foreign governments to try and detect, and then collect, messages from possible al Qaeda sources. The current warnings come from a general increase in al Qaeda messages, and bits of data that has been read from some of those messages. This is classic traffic analysis. It's not perfect, but in past wars it has provided valuable and life saving information. But because it's imperfect information, you can expect a lot of false alarms. And al Qaeda is suspected of knowing how to manipulate message traffic in order to deceive our traffic analysis methods. This form of deception has also been used in the past with success. For example, when the June 6, 1944 D-Day invasion of France was being planned, a fake army group headquarters was set up in England. Actually, this "army group" consisted largely of a lot of radio and telegraph operators sending a large number of messages that would be normal if the "army group" were preparing for an invasion of France, in an area other than where the actual invasion was going to take place. The deception worked. German traffic analysis experts were fooled and the Germans believed the actual invasion at Normandy was just a feint, a move to get the Germans to send their reinforcements to Normandy rather than were the fake "army group" was going to invade.