More than 18,000 law enforcement agencies were again placed on the highest state of alert last night as the FBI issued a new warning of terror attacks against America or American targets overseas within the next week.
Judging that the need for awareness outweighed the risk of panicking an already jumpy nation, President George Bush agreed that the warning was credible enough to be made public, even though there was no specific information about possible targets or methods.
At a hastily arranged press conference last night, the US Attorney General, John Ashcroft, said: "There may be additional terrorist attacks within the United States and against US interests over the next week. It's not specific but it is information we think the American people have the good, mature judgement to appreciate and understand.
"We urge Americans in the course of their normal activities to remain alert and to report unusual circumstances and inappropriate behaviour," he said.
It is the second such warning since the terror attacks on New York and Washington that killed thousands. Again, when that warning was issued on 11 October, there were no details available regarding possible targets or methods.
The FBI director, Robert Mueller, said last night it was not clear whether that warning had been related to the spate of anthrax mailings, which have killed three people, infected more than a dozen others and created genuine anxiety, especially within Washington.
But he said it was very possible that the alert that was issued helped in some way to avert a terror attack.
While no information was given about the intelligence received, the presumption in Washington last night was that the alert was linked to the 11 September attacks. One senior official said the alert was "most likely" related to al-Qa'ida.
Every day a CIA document with the highest security classification, called the "Threat Matrix", is passed to the top national security and intelligence officials. It presents the freshest and most sensitive raw intelligence on dozens of bomb threats, hijackings or poisonings. Only threats deemed credible are included. The Washington Post reported that one day three weeks ago, the Threat Matrix contained 100 threats to US facilities at home and around the world.Reuse content