The FBI has "specific and credible" information that a fresh terrorist attack is planned on the US within the next week, the US Attorney General, Mr John Ashcroft, said last night.
But Mr Ashcroft said the intelligence received did not reveal the target or the method of the attack, as he put the US on the highest state of alert. The alert was the second to be issued this month and Mr Ashcroft said it was intended to warn police forces and government agencies across the country to be ready.
"The administration had concluded, based on information developed, that there may be additional terrorist attacks within the United States and against United States interests over the next week," said Mr Ashcroft.
"The administration views this information as credible, but unfortunately it does not contain specific information as to the timing or target."
Speaking shortly after Mr Ashcroft's announcement, a US official said the warning was "most likely" linked to the al-Qaeda network of Osama bin Laden. The official asked not to be named.
Mr Ashcroft's warning went on: "We have notified law enforcement to continue on highest alert and to notify the FBI immediately of any unusual or suspicious activities. We ask for the patience of the American people if and when they come across the increased precautions being taken.
"We urge Americans in the course of their normal activities to remain alert and to report suspicious or inappropriate behaviour to the appropriate authorities."
Mr Ashcroft said bodies including the Federal Aviation Administration, the departments of transport and energy and the Nuclear Regulatory Administration had been put on the highest state of alert tonight in anticipation of the threat. Mr Ashcroft called for people to go about their normal lives, and said they should be tolerant of the additional restrictions they might meet.
The director of the FBI, Mr Robert Mueller, said the information was being released because it was believed to be credible.
"On every occasion when the government has received credible information we have chosen to warn our colleagues in the law enforcement community and we have chosen to do so even though it is not specific as to intended target or intended method," said Mr Mueller.
"I know how difficult it is for officers they represent and all other state and local officers to respond without greater detail. We are again asking them and through them, local communities, to remain extremely vigilant."
Mr Mueller said he believed telling the American people of the warning would be a "force multiplier" which would help disrupt any planned terrorist activities.
He refused to say how the information had been obtained, but he said: "When we have received this additional information specific as to timescale, not specific as to other details, we think it is important to put it out there."
A previous warning, issued on October 12th, had said there was a "credible" threat of an attack on America, and had put police and other government agencies on the highest alert. Last night's warning renewed that state of alert and meant many police would return to 12-hour shifts and cancelled holidays for at least the next week.
Other precautions taken in the wake of the alert earlier this month had included heavily armed police officers outside government buildings and renewed air patrols over potential targets, including nuclear power plants and major cities.