A new Defense Department study casts serious doubt that combat stress is behind the recent spike in suicide rates by members of the military. The study found the suicide rate had increased more for those who hadn't deployed to combat zones than those who had.

The project, which began in 2008, tracked 1.6 million soldiers and interviewed 10,000 before and after their deployments. The results will appear in the Journal of the American Medical Association on Wednesday, according to the Wall Street Journal.

After the military actions in Iraq and Afghanistan, numerous news reports pointed to increases in suicide rates by members of the military. In 2004, the military rate was half that of the general population. By 2009, it had more than doubled, and exceeded the civilian suicide rate.

Post-traumatic stress disorder from having served in combat was widely presumed to be the culprit. But the study found that although suicide rates for soldiers who had been deployed had doubled, the rate for those who had not had tripled.

"The story is not so simple as 'war is hell,' and you send people to war and bad things happen to those people," Dr. Michael Schoenbaum, a senior mental-health adviser with the National Institute of Mental Health and a survey leader, told the Journal.

What is driving the rates up? The answers appears to be that more people who already have mental health problems are joining. The study found that one-third of the troops who attempted suicide had a mental disorder before they entered the service.