Federal officials said the alert should continue at least through the end of the Islamic holy month of Ramadan. Homeland Security Director Tom Ridge did not mention Ramadan specifically but said such threats had come up before around "important religious observations in other faiths."
"Now is not the time to back off," Ridge said. "Obviously, the further we're removed from Sept. 11, the natural tendency is to let down our guard. We cannot do that. We are a nation at war."
Ramadan's end, which can vary from country to country, will come in mid-December.
Monday's warning that went out to 18,000 law enforcement agencies around the nation was the third issued by the FBI. The agency issued two others on Oct. 11 and Oct. 29.
Ridge did not cite a specific threat, but said U.S. intelligence had seen an increase in the volume of information warning in general of more attacks.
"The information we have does not point to any specific target either in America or abroad and it does not outline any specific type of attacks," Ridge told reporters, saying the quantity and level of threats "have reached a threshold where we should once again place the public on general alert."
"When there's a volume and a decibel level and a credibility attached to them, the president said, even though there's not a specific location or a time frame...the public needs to have confidence that we will share this information with them and that's exactly what we did," he told anchor Bryant Gumbel.
Mr. Bush reviewed the analysts' conclusions Monday morning, and "he approved our decision to go forward and make the announcement," Ridge said.
Asked if he worried that Americans were becoming jaded by the repeated alerts, Ridge said the process of warning the public "is an art, it's not a science. There are shadowy soldiers. This is a shadowy enemy."
The FBI issued the earlier alerts in the days after the U.S. military launched airstrikes in Afghanistan Oct. 7, targeting Osama bin Laden and his Taliban protectors. Bin Laden is the prime suspect in the Sept. 11 attacks.
Ridge urged citizens to view the current alert as "a signal to be vigilant" and asked that they report suspicious behavior to local authorities.
However, "the president said it's not a signal to stop your life; it's just a reminder to be alert. Be on guard," he said.
He said officials took the "convergence of different religious observations" this month into account when they decide another alert was in order.
Meanwhile, law enforcement officials told The Associated Press that investigators have gathered evidence showing similarities among the last three terrorist attacks against Americans by bin Laden's supporters.
Those attacks include the Sept. 11 suicide hijackings, the October 2000 bombing of the USS Cole in Yemen and the 1998 bombings of two U.S. embassies in Africa.
The officials, speaking on condition of anonymity, said the similarities included the way the attacks were planned, communicated and carried out, and the way the attackers were trained. The officials declined to be more specific.
"There are certainly similarities among the three, some of which have emerged more clearly in the last few weeks," one official said.
The investigators said they also are examining whether some of the same people were involved in planning and assisting the three attacks. One official said authorities are waiting for more information from authorities in Yemen and other countries about certain suspects.
The Washington Post Tuesday reported Osama bin Laden and his al Qaeda network may have made greater strides than previously thought toward getting the information to make a "dirty bomb," a crude radiological weapon that would use conventional explosives.
The newspaper said the conclusion was made after interrogation of captured al Qaeda members and evidence gathered in the last month at al Qaeda facilities in Afghanistan by CIA officers and U.S. special forces.
Ridge denied that was the reason for the latest alert.
"We know that bin Laden has been looking for...all kinds of weapons of terror over the past several years...but this has absolutely nothing to do with the possibility that he has secured any particular kind of weapon," he said on the Early Show. "Again, the threat assessment wasn't based upon any specific information; about a kind of weapon, a location. It was just a lot of activity, a lot of noise, decibel level's up."
Tens of thousands of letters mailed around the country may have picked up trace amounts of anthrax in a New Jersey postal facility, but Centers for Disease Control officials and Ridge say the worst of the threat appears to be over.
"As time goes by and we don't see any indications in the public health system that there has been any sign of inhalation anthrax other than the two regrettable cases that we have in New York and Connecticut, other than the Postal Service workers here in Washington, D.C....as we get further and further away from the [letters sent to Capitol Hill], I think it's really good news in the long-term for all of us," he said.
Ridge admitted that issuing alerts where nothing happens afterwards could diminish the impact of future alerts.
"Hopefully, the public will be reassured that when we have that kind of information, we share it with them," he said.
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