The flashing blue-and-yellow box, headlined "Alert Notification," popped up over the e-mail on Peter Murphy's computer screen at the Pentagon, startling the Marine Corps commandant's counsel.

"It scares the bejabbers out of you," Murphy said.

It was another warning -- the second he received Friday morning -- from the Pentagon's Computer Emergency Notification System. This one told workers to obey all instructions from emergency workers in the event of an attack. "A crisis is no time to second-guess," the alert lectured.

Terrorism is not a theoretical concern at the Pentagon. The military's headquarters, host to 24,000 workers, was a target of the al Qaeda terrorist attack of Sept. 11, 2001. A hijacked jet was flown into the west wall, killing 125 people at the Pentagon as well as 59 passengers and crew members and five hijackers aboard the plane.

The heightened terror alert of the past week is being taken with deadly seriousness in the building, particularly in Murphy's office, which was destroyed in the attack.

"It's hard not to be concerned -- we crawled out of here last time," said Robert D. Hogue, the deputy counsel. Hogue would have been killed had he not walked out of his office moments before the plane struck. As it was, the force of the blast cost him the hearing in his left ear.

The computer emergency notification system, set up after the Sept. 11 attack but used only occasionally, flashed alerts to Pentagon workers several times last week. Workers were greeted Friday with fliers containing a checklist of what to do in an emergency. And Defense Department police plan a town hall meeting this week to review security with employees.

Workers around the building are well aware that terrorists have struck again at targets that were not destroyed in a first attack. The World Trade Center in New York, target of a bomb in 1993, was destroyed in the Sept. 11 attack.

"People here almost assume that the Pentagon is going to be a target," Murphy said.

"People as a whole, their anxiety level is up," said Paul Gonzales, a supervisor in the Defense Intelligence Agency's comptroller's office.

After the plane struck, seven of the 18 workers in Gonzales's office died and five were hospitalized, including Gonzales, who had critical lung injuries.

The survivors still mourn their lost co-workers. The threat of more terrorism has left them jumpy. "We hear noises to this day that scare the heck out of you," Gonzales said. But the experience also left them with a sense of peace, knowing their fate is not entirely in their hands, he said.

"We are where we are," said Gonzales. "For a lot of us, it was a matter of seconds whether we lived or died."

Stress levels are high for many Pentagon employees, said Sandi Hanish, a clinical nurse specialist with Operation Solace, a stress management team of physicians, social workers and psychologists set up by the Army after the Sept. 11 attacks.

"These are citizens like everybody else -- they haven't been vaccinated against social concerns," Hanish said.

The heightened terrorist alert is only one of many elements contributing to stress at the Pentagon. Another major cause is the long hours many are working as the military prepares for war with Iraq, Hanish said.

The nature of the work at the Pentagon, including the access some have to classified information about such things as chemical or biological weapon threats, is another factor. "I'm sure a number of them have too much information for their peace of mind," Hanish said.

Anxiety over a possible terrorist attack is particularly high among civilian workers at the Pentagon, some employees said. "A lot of military people, this is what they prepare for," said a civilian defense official, who declined to be identified. "For civilians, it's a little frightening."

As much as possible, life continues as usual. In the Marine commandant counsel's office, workers gathered around the table Friday in the conference room for pizza and a chocolate carrot cake baked by Murphy's wife to bid farewell to a colleague.

On the Pentagon concourse, officers in uniform browsed for chocolates and flowers for Valentine's Day.

But memories of Sept. 11, 2001, are never far away. In a corridor off the concourse, workers stopped in to review display models of the six finalists selected through a competition to design a Pentagon memorial to the victims of that attack.

For many, it was an emotional experience, made more so by the realization that those victims might not be the last. "Most come because they knew someone who was lost, or were hurt themselves," said Carol Anderson-Austra, an Army Corps of Engineers landscape architect who is helping oversee the competition. "Now, it's even more sensitive."