A large study of nearly 4 million U.S. service members and veterans has found that deployment to Iraq and Afghanistan is not associated with an increased risk of suicide.

Appearing in JAMA Psychiatry online on Wednesday, the study by researchers at the Defense Department's National Center for Telehealth and Technology, or T2, indicates that although the suicide rate among active-duty personnel has increased since 2001, the rate for those who deployed to a combat zone was roughly the same as for those who did not.

Rather, the study found that the military group at highest risk for suicide are those who served in the military for less than a full enlistment.

In fact, the suicide rate among those who served less than a year was 2.5 times the active-duty rate, according to the research.

And those rates remained extremely high among those who served less than three years.

"This is an important finding. It shows those who separated from military service had a 63 percent higher suicide rate overall. ... Why are these people at higher risk, we don't have data to explain it," said Mark Reger, study lead author and deputy director of T2.

But the authors did speculate, based on previous research on why people commit suicide, that problems such as injury, a legal issue or mental health conditions that might force a person out of the military could contribute to suicidal behavior.

Or, the transition itself — the loss of identity, difficulty developing a new social support system, issues trying to find meaningful work after service or a sense of feeling like a burden or they don't fit in to civilian society, could play a role.

"Additional research is needed to clarify what the circumstances are surrounding early discharges and how these factors may be related to suicide," the authors wrote.

To conduct the study, researchers reviewed all troops who served from Oct. 7, 2001, to Dec. 31, 2007. Service history was obtained from the Defense Manpower Data Center and researchers reviewed death records from the Armed Forces Medical Examiner system as well as the National Death Index.

Of the 3,945,099 personnel in the study, 31,962 people died during the six-year study, 5,041 documented suicides.

Of the suicides, 3,879 were service members who did not deploy and 1,162 were those who did participate in OEF or OIF.

Those figures translate into a suicide rate of 17.78 per 100,000 person years for those who did not deploy and 18.86 per 100,000 person years for those who did — a difference that is not considered statistically significant.

Multiple deployments appeared to influence the rate somewhat, with those who deployed more than once experiencing a rate of 19.92 per 100,000.

Among those who separated early, however, the rate difference was significant. Those who separated from the military without having deployed had a nearly rate of 26 per 100,000 person years rate and those who had deployed had a rate of 26.48 per 100,000 person years.

Person years are a statistical measure of the number of years that members of a population have been affected by a particular condition multiplied by the number of members in that population.

It often is used in military epidemiological studies to account for the changing numbers of personnel serving in the military each year.

The civilian rate, adjusted for age, gender and socioeconomic factors similar the the military population, is 18.8 per 100,000, according to Army and National Institutes of Mental Health calculations.

Subgroups at highest risk, besides those who had served less than a year, included Marines who did not deploy and separated from the Corps early, with a rate of 32.6 per 100,000, and Army soldiers who deployed and separated — 28.1 per 100,000.

Reger said the researchers did not have access to medical records, personnel records or mental health information and would need that information to further understand the reasons why those who leave military service early are at high risk.

"We also would like to look at factors such as combat exposure — obviously everyone who deploys does not see the same level of combat, and that may have an impact, as do combat injuries or other factors," Reger added.

The research could help direct future suicide prevention programs and has implications for services for those leaving the military, especially at-risk veterans discharged under less-than-honorable circumstances.

Under Veterans Affairs Department policy, all Iraq and Afghanistan veterans are eligible for up to five years of medical care through the VA if they have been discharged under other-than-dishonorable conditions.

But those receiving a dishonorable or bad-conduct discharge or dismissed under questionable circumstances may not be eligible for VA care and benefits, including health services and transition assistance.

The data suggest that considering prevention options for those who have discharges that were not honorable may be helpful, Reger said.

Reger said he also hopes the study can clear up any misconceptions that combat deployment puts military veterans at risk.

"if can be a disservice to service members to misrepresent the nature of the population. We need to be clear that the deployed force overall adjusts well and is not at increased for suicide," he said.