March 21 (HealthDay News) -- In the years after the 2001 World Trade Center terrorist attack, the rate of psychiatric illness among children who lost a parent in that event more than doubled -- from about 32 percent before the attacks to almost 73 percent after, a new study finds.

More than half of the 45 bereaved children, average age 9, in the study suffered from some form of anxiety disorder, such as post-traumatic stress disorder (PTSD), said researchers at New York-Presbyterian Hospital/Weill Cornell Medical Center in New York City. About 30 percent of the bereaved children developed PTSD, a rate that is 10 times higher than that seen in a control group of 34 non-bereaved children.

The study found that 27.3 percent of bereaved children had separation anxiety, and 25 percent had generalized anxiety, double the rate of non-bereaved children. The rate of simple phobias among the bereaved children was pegged at 13.6 percent compared with 5.9 percent of non-bereaved children.

The rate of major depressive disorder among bereaved children was 13.6 percent, compared to 5.9 percent among non-bereaved children.

The researchers also found that many children who lost a parent in the WTC terrorist attack had chronic, heightened activity of the brain's "stress-response system."

"Continued activation of this system can lead to long-term hypersensitivity to stress as adults and even impact on bone health, since the stress hormone cortisol can harm bone," lead researcher Dr. Cynthia Pfeffer, professor of psychiatry at Weill Cornell Medical College and attending psychiatrist at New York-Presbyterian Hospital/Weill Cornell Medical College, said in a prepared statement.

The children in the study, conducted at five sites across the greater New York metropolitan area, were recruited from four months to three years after 9/11. After they were enrolled in the study, they were assessed once every six months for two years.

The findings are published in the April issue of the journal Biological Psychiatry.