The attacks of September 11 have left a lingering psychological impact on the American nation, research suggests.

More than one in six people in the US living outside New York City reported symptoms of post-traumatic stress in the two months following the attacks.

Research suggests that 6% continued to be plagued by problems six months after the strikes.

Some symptoms may be a normal response to an abnormal event

Dr Sandro Galea

Many people were particularly distressed about the threat of future terrorist attacks.

The researchers, led by Professor Roxane Cohen Silver, of the University of California at Irvine, had launched a study into mental health prior to the events of September 11.

This meant they were extremely well placed to track the pyschological effects of the attacks, both in the immediate aftermath, and in the longer term.

Professor Silver said: "Our data show that six months after the events of 9/11, the effects continued throughout the country among individuals who were, for the most part, not directly affected by the attacks."

Widespread impact

She said the results showed it could not be assumed that the people who were most likely to suffer the most severe psychological problems were those directly affected, or exposed to the events.

"We believe it is important for health care professionals to recognise that potentially disturbing levels of trauma-related symptoms can be present in a substantial portion of individuals who are not directly exposed to a trauma, particularly when the trauma is a massive national tragedy such as the 9/11 attacks."

The people who were most likely to report longer-term psychological problems were those who either "gave up" or refused to believe what had happened on September 11.

Subjects were questioned about their mental state during the first two weeks, two months and six months after the attacks.

'Normal response'

A total of 933 people participated in the first and second rounds of the survey, and 787 of those participated in the third round.

Dr Sandro Galea, of the New York Academy of Medicine, has conducted research into the psychological impact of the attacks on local residents.

He told BBC News Online that the symptoms of post-traumatic stress disorder were more common among New Yorkers, but that they appeared to be lessening with time.

"Some symptoms may be a normal response to an abnormal event," he said.

"A lot of psychiatric services have been made available to people, and it is my impression that those who needed to access the services have been able to do so."

The research is published in the Journal of the American Medical Association.