U.S. officials detected that the group took immediate notice of the escalation to Condition Orange and all the added scrutiny that brings, but said today there's been no letup so far in intelligence signs indicating another attack is on the way.
Meanwhile, New York officials continue to pile on added layers of security, including a police command center exercise. And crucial industries were issued warnings to look for "rogue" employees.
"You'll be seeing National Guard people at subway stations in New York. It's not cause for alarm. It should be cause for reassurance," said Gov. George Pataki.
The FBI and the National Infrastructure Protection Center said in the bulletin that security routines should be varied and officials should think back to unusual incidents that might indicate they are being targeted.
In addition, the warning said officials should check Internet sites describing their facilities and "consider how that information might assist terrorists interested in planning an attack. Operatives will likely research potential targets extensively prior to an attack."
The bulletin, issued Wednesday, also said officials should examine whether there are threats from "insider" employees who may have been sent to infiltrate their facilities.
"Planning may begin months or years before an actual terrorist attack," the bulletin said.
The FBI advisory is the latest in a string of warnings from the government this week, which have helped to put some American cities on their highest state of alert since the Sept. 11, 2001 attacks.
Last week the White House increased the nation's terror threat status to "high," and CIA director George Tenet said Tuesday that an attack was possible this week, perhaps involving a "dirty" radiation bomb.
Intelligence officials, meanwhile, are examining yet another purported message from Osama Bin Laden and say the tape is "not inconsistent with his other messages." The latest is 50 minutes long and contains "no reference to the crisis with Iraq."
And further analysis of a tape whose contents were revealed Tuesday by Sec. of State Colin Powell suggest that it was made several weeks ago. Officials in Qatar today say they were the ones who slipped the U.S. an advance copy of that message.
Signs of heightened alarm were visible on streets and in stores.
Air defense systems were placed around Washington, and New York City police stopped suspicious vehicles. Americans stocked up on duct tape, plastic sheeting and other supplies.
FBI personnel assigned to rapid response teams that would react to any terrorist attacks have been told to have a bag packed for three days' deployment and put on standby. And U.S. Capitol police were told to carry gas masks at all times.
And - as is typical with heightened security - there were several scares around the country Thursday, including one at an Atlanta suburban high school where a device with a timer was found and at the Dallas-Ft. Worth airport where three concourses were evacuated after an emergency exit door was found ajar.
A CBS News/New York Times poll finds eighty-two percent of those interviewed on Monday and Tuesday believe a terrorist attack in the next few months is likely — the highest figure since late 2001.
And concern appears to be increasing.
Just a month ago, only 14 percent thought another terrorist attack was very likely. That number has risen to 35 percent now. The percentage also rose as the polling took place; there was a sharp increase Tuesday in the number who said they feel an attack is very likely.
The poll also found that as many people say the government's warnings make them feel anxious as say the warnings make them feel more secure.
TERRORIST ATTACK ON U.S.
Very likely 35%
Somewhat likely 47%
Not very likely 16%
Very likely 14%
Somewhat likely 48%
Not very likely 35%
But a source tells CBS News the Homeland Security Department has become increasingly worried that most people outside the cities hit on Sept. 11 are not taking the terror threat seriously.
So the department is now said to be working on a plan to carry its message directly to the nation's heartland, warning that anything can happen anytime, anywhere.
In testimony before the Senate Armed Services Committee Wednesday, Tenet warned that an attack could occur as early as this week, either in the U.S. or on the Arabian Peninsula where American troops are stationed.
"If given the choice, al Qaeda terrorists will choose attacks that achieve multiple objectives, striking prominent landmarks, inflicting mass casualties, causing economic disruption and rallying support through shows of strength," the CIA director said. "The bottom line here, Mr. Chairman, is that al Qaeda is living in the expectation of resuming the offensive."
Tenet reminded senators that twice this fall, messages from bin Laden were followed by terrorist attacks — the Bali nightclub bombing and the attack on a hotel in Kenya.
In other testimony this week, Tenet and FBI director Robert Mueller said the intelligence did not provide a specific date, time or place for the attack, but hinted that one might be coordinated with the end of the Muslim pilgrimage, or hajj, this week.
Both men said the latest threat was an example of why al Qaeda remains the greatest terrorist threat to the United States, one that could have several "sleeper" agents already in the country.
Privately some officials say they regret the public's been put through such an emotional ringer during the latest threat crisis, but insist they're doing it for the right reasons. Al Qaeda has always been mindful of increased security, they point out, and anything that disrupts their plans is worth the anxiety.