The study, published in the Journal of Traumatic Stress, finds residents who were injured during the attacks were the most likely to develop PTSD.

The findings released by the New York City Health Department's World Trade Center Health Registry show that Lower Manhattan residents developed PTSD at three times the usual rate in the years following the attacks. The rate among residents -- 12.6 percent -- matched the rate previously reported among rescue and recovery workers -- 12.4 percent.

The study, based on surveys of 11,000 residents from the World Trade Center Health Registry, is the first to measure the attack's long-term effect on the mental health of community members, Dr. Thomas R. Frieden, New York City health commissioner says.

Aside from injured residents -- 38 percent of whom developed symptoms of PTSD -- the most affected groups were those who witnessed violent deaths and those caught in the dust cloud after the twin towers collapsed.

Roughly 17 percent suffered PTSD in each of those groups. The symptoms most commonly reported were hyper-vigilance, nightmares and emotional reactions to reminders of Sept. 11, 2001.