(Reuters Health) - Many civilian survivors of the September 11 attacks on the World Trade Center were still suffering from symptoms of post-traumatic stress several years after the 2001 disaster, a new study finds.

Surveys of nearly 3,700 people who escaped the Twin Towers that day found that nearly all -- 96 percent -- still had at least one symptom of post-traumatic stress disorder (PTSD) two to three years later.

And of those, 15 percent screened positive for full-blown PTSD -- a rate about four times higher than that seen in the general population in any given year.

The study, reported in the American Journal of Epidemiology, is the first to focus on the long-term mental health of the people who were actually in the Twin Towers on the morning of September 11.

Past studies have looked at the general public, or people who lived near the World Trade Center, said senior researcher Dr. Sandro Galea, of Columbia University in New York.

That's because at first, researchers had no good way of finding and studying survivors who escaped the buildings that day. But government officials have since developed a health registry that includes a large sample of people who were in the World Trade Center during the attacks.

Galea said his team's findings help pinpoint those survivors who appear to be at particular risk of long-term PTSD.

Not surprisingly, the risk rose along with the severity of the trauma people went through on September 11.

The study found that people who had escaped from floors above the planes' "impact zone" were at greater risk of PTSD than those who escaped from lower floors. Similarly, people who were evacuated relatively later, or who had to run from the cloud of debris sent out by the collapsing towers, were also at elevated risk.

In addition, survivors who were injured or who personally witnessed a "horror," like people falling or jumping from the towers, were at increased risk.

Of survivors who said they witnessed a horror, for example, 16 percent had probable PTSD two to three years later, versus four percent of those without such experiences.

All of this, Galea said, has implications for future disaster planning.

"This tells us, for example, that any delay in evacuation matters," he said, noting that survivors who did not start evacuating after the first plane hit were at relatively greater risk of PTSD.

"This also tells us that you want to get people clear of the area quickly, so that they don't have to witness any more bad things," Galea said.

But the researchers found that the biggest predictor of long-term PTSD was not anything related to the disaster itself. Instead, it was income.

More