The Department of Homeland Security has stressed that the new "high" alert does not mean Americans should stop traveling. But by heightening the terror alert to orange this week, the government may have given travelers a reason to avoid airports and major metropolitan areas.
AAA, the Heathrow, Fla.-based travel agency, said the government's decision to raise the terror alert to orange on Tuesday would not affect its prediction from last week that 84 percent of all Memorial Day vacations will be by car and 11 percent by air.
"If something serious happens on our soil, then conditions could change," AAA spokesman Jerry Cheske said.
The Homeland Security Department's orange alert suggests a high threat of terrorist attacks, and authorities extended security measures around the country. Orange, or "high" alert, is the second-highest alert level.
At the same time, the Pentagon's Defense Intelligence Agency has now raised its terrorist threat warning for military bases in the U.S. to its highest level.
The increased alert was ordered in the wake of bombings last week in Riyadh, Saudi Arabia, and Casablanca, Morocco, as well as intercepts of threatening communications hinting at possible imminent strikes.
Thirty-four people, including eight Americans, died in the Riyadh blasts. The attacks in Morocco killed 29.
The FBI said Wednesday that the bombings could be a "possible prelude" to a terrorist attack in the United States and that attacks are "likely" against U.S. and Western interests abroad.
The warning was contained in an FBI bulletin sent weekly to more than 17,000 law enforcement agencies nationwide, marking the third time in six days the FBI has urged state and local police to increase their vigilance.
FBI Director Robert Mueller and other officials acknowledged Wednesday the information was nonspecific, pointing to no particular time, target or method of attack.
But officials said they consider a new communication, purported from bin Laden's top deputy, Ayman al-Zawahri, as further evidence of possible terrorism. Previous messages from al Qaeda leaders have sometimes heralded new attacks.
The message called on Muslims to "consider your 19 brothers who attacked America in Washington and New York with their planes as an example."
The speaker also singled out the United States, Britain, Australia and Norway. In response, the U.S. Embassy in Oslo, Norway, announced it would be closed to the public on Thursday.
Britain, Germany and Italy joined the United States in closing diplomatic offices in Saudi Arabia for at least a few days starting Wednesday because of terror fears.
Security was tight in Riyadh, where officials say they are working to avert more violence, but cannot guarantee attacks have ended.
Saudi police arrested four more suspects linked to al Qaeda in their crackdown on extremists following those bombings, a Western diplomat said Thursday. Four suspected al Qaeda associates were arrested this weekend in the same probe.
On Tuesday, Saudi security officials said three suspected al Qaeda militants had been arrested a day earlier in the western port city Jiddah, and later reports — denied by the Saudi interior minister — linked those three to a possible hijacking plot.
Also Thursday, Iran demanded that Washington prove its claims that Tehran harbors al Qaeda terrorists.
On Wednesday, Defense Secretary Donald H. Rumsfeld and State Department spokesman Richard Boucher accused Iran of harboring al Qaeda members.
"There's no question but that there have been and are today senior al Qaeda leaders in Iran, and they are busy," Rumsfeld said.
U.S. officials have identified five senior al Qaeda operatives who they believe have been to Iran since the U.S.-led coalition ousted the Taliban from neighboring Afghanistan.
They include Saif al-Adil, an Egyptian described as bin Laden's security and intelligence chief; Saad bin Laden, one of the al Qaeda leader's sons; and Abu Musab Zarqawi, the operational commander who Washington accuses of ties to Saddam Hussein.
It's the second time since invading and defeating Iran's neighbor, Iraq, that the United States is putting pressure on Tehran. The U.S. earlier claimed Tehran was interfering in postwar Iraq by manipulating Shiite opinion.
Saeed Pourazizi, a close aide to President Mohammad Khatami, said it was Tehran's policy to crack down on al Qaeda — not support it.
Al Qaeda "is a terrorist group threatening Iran's interests. Its extremist interpretation of Islam contradicts the Islamic democracy Iran is trying to promote. There is no commonality of anything between us," Pourazizi told The Associated Press.
Iran is a Shiite Muslim-dominated state, while bin Laden's al Qaeda group preaches a hard-line interpretation of the Sunni sect of the Islamic faith.
After the Soviet pullout from Afghanistan, Iran opposed the Taliban, which eventually allied with al Qaeda.