Researchers studying the emotional aftermath of the Sept. 11, 2001, attack on the Pentagon reported yesterday that depression and post-traumatic stress remained significant two years later in an office of military and civilian employees who lost two dozen of their colleagues.

The degree of continuing psychological upheaval was greatest among those who were injured that horrific morning, when a commandeered American Airlines plane was crashed into the Pentagon's western flank, killing 184 people on the flight and on the ground.

But even among those not physically harmed, more than one in six were still struggling to cope -- an indication that their distress had settled into a chronic pattern.

The research illustrates the "enduring impact of terrorism" and will help doctors identify the types of individuals at greatest risk for mental health problems in the event of future strikes, the authors said. The findings also point to the need for ongoing mental health treatment, they added.

"Leaders in government and business, as well as disaster medical planners, should ensure that high-risk populations . . . have long-term access to mental health services," they concluded.

The study was presented at an international conference on military medicine being held this week in Crystal City, just days after the third anniversary of Sept. 11. Led by psychiatrist Thomas A. Grieger of the Center for the Study of Traumatic Stress -- part of the Uniformed Services University of the Health Sciences in Bethesda -- it builds on previous surveys of other Pentagon employees at seven and 13 months after the attack.

The latest examination focused on a management office at the Pentagon that lost more than 10 percent of its staff on Sept. 11, 2001, and then dealt with major relocation and reorganization issues. The office was not identified, in keeping with standard privacy practices in research.

"This particular work group was hit particularly hard," Grieger said. In the weeks after the attack, its staff was provided with extensive mental health services, and many availed themselves of the help. Nearly three-quarters of the workers suffering from depression or post-traumatic stress disorder sought care at least once during the two years, and a third remained in treatment during the second year.

Grieger considers the numbers "quite positive," especially given the enduring stigma about mental health care within the military. "There's an internal limitation on their acknowledgment of their vulnerability," he said.

But of the 267 people who responded via anonymous online surveys inquiring about their emotional well-being, some were still having problems 24 months after the attack. (The respondents were divided fairly evenly between military, most of them officers, and civilians, who included secretaries to executives.)

Not surprisingly, those not at the Pentagon that day reported less psychological distress or psychiatric illness. The effect of indirect exposure -- through at least three hours of television coverage of the destruction there and in New York -- was no longer measurably statistically significant after two years.

The situation was far different for the 129 men and women who had been in the building when the plane hit and who escaped through burning offices and collapsed hallways, sometimes stepping around dead colleagues as they fled. About one-fourth of this group continued to experience symptoms of depression or post-traumatic stress. Among those with physical injuries, the rate was double that.

Among workers who saw injured or dying victims or interacted with grieving families -- even if they had initially been elsewhere on Sept. 11 -- a third still experienced depression or post-traumatic stress symptoms two years later.

Grieger would not speculate on how Pentagon employees might be faring at the three-year mark, though he plans to follow up this fall. Their recovery may be complicated by the fact that "they've been going back to work in the place where they were attacked." There, memorial displays and personal mementos and every new terrorist alert warning inevitably have more resonance.

"It's difficult to predict," he said.

Two survivors of the terrorist attack on the Pentagon talk about what happened on Sept. 11, 2001. The study focused on the Pentagon attack.