The September 11 terror strikes left American psychiatrists a lasting legacy of unexpected size: thousands of people living and struggling with post-traumatic stress disorder, 10 years on.
An eye-opening example of the PTSD on view in New York's daily life a decade after the attacks: last Tuesday, when a relatively rare earthquake was felt up much of the US east coast including in New York City, locals spontaneously evacuated high-rises here, pouring into the streets in distress.
"The first thing to come to their minds was 'Oh, my God, is there another bombing, what's going on?'" said Dr. Jacob Ham, who leads the program for Healing Emotions and Achieving Resilience to Traumatic Stress at Beth Israel Medical Center and St. Lukes Roosevelt Hospital Center.
In fact, "a lot of people were having PTSD reactions without understanding it. They were having a physical response, they were panicking more than the (real) situation might make them, to react in this way," he explained.
PTSD is an anxiety disorder that can follow a psychologically traumatizing experience, sometimes immediately but at times months later.
It is considered a full PTSD case when anxiety lasts longer than a month; a shorter case would be deemed acute stress disorder. The mental health definition of PTSD also notes that the patient's fear is felt as present, even when the victim is not physically or psychologically threatened.
With a decade of fighting in Iraq and Afghanistan, PTSD has skyrocketed among the US military ranks.
It can happen to victims of violent crime or domestic abuse, but PTSD also can be common in the aftermath of a major terror attack such as 9/11.
The pathology as it is described now was developed in 1978 by Charles Figley in a book about Vietnam war veterans plagued by their traumatic experiences at war.
New York's health department has just started a third study to look into the physical and mental health of the more than 70,000 people directly exposed to the 9/11 attacks.
"Only now has sufficient time passed to begin investigating potential late emerging and long-term health effects associated with (World Trade Center) exposure," explained Health Commissioner Dr. Thomas Farley.
"Responses to the third survey will provide researchers with health updates that can help guide clinical services in the future."
According to official New York city figures, at least 10,000 firefighters, police officers and civilians exposed to the terror strikes have experienced PTSD, the most common psychiatric disorder stemming from the 9/11 attacks.
New York Health Department projections based on those figures are that some 61,000 of the 409,000 people who were in the area at the time of the attacks probably experienced PTSD in the first six years after 9/11.
For Roxane Cohen Silver, an expert on PTSD at the University of California, "the attacks of 9/11 did far more than destroy buildings and kill thousands of innocent people. They interrupted routine patterns and tugged our social fabric" in the United States, she said.
"Even those individuals who did not know anyone who died that day have been touched by the tragedy. We are different now... We sometimes gaze askance at young men carrying backpacks on public transportation," she added.
Beth Faitelewicz, a nurse at Beth Israel hospital for the past 25 years, lives, with her husband and daughters, close to the site of the attacks in New York. She says that her husband has suffered PTSD, which has put strain on her.
"I never went for help. I should go, maybe, but I never went. I try to deal with things on a daily basis. I've got to be strong, Im the mommy," she told AFP.
"But if one person in the family is affected, we are all affected."