The suicide rate for veterans of the Iraq and Afghanistan wars has risen sharply in recent years, and at times rivaled the death rate for soldiers in combat. At the moment it is the second leading cause of death for these veterans, according to a new study, which also suggests that this increase in suicide is linked to the head injuries that are all too common among vets returning from these combat zones.

Soldiers who sustained multiple traumatic brain injuries (TBI) appear to be at much higher suicide risk than soldiers who sustained fewer such injuries. TBI is now unfortunately known as the "signature injury" of the Iraq and Afghanistan wars.

None of the veterans without head injuries had these thoughts of suicide.

The number of TBIs a soldier sustained during combat affected his or her likelihood of having thoughts or plans of suicide years later. The team focused on 161 former soldiers in Iraq, and classified them as having zero, one, or more than one TBI.

The veterans were also asked about psychological symptoms they’d experienced over the years, like depression, PTSD, and suicidal thoughts, which served as an indicator of their risk for actual suicide.

The results: over 21% of the veterans who had sustained more than one TBI had had suicidal thoughts or ideas in the past; in contrast, about 7% of veterans who had sustained exactly one TBI had suicidal thoughts, and none of the veterans without head injuries had these thoughts.

“That head injury and resulting psychological effects increase the risk of suicide is not new,” said study author Craig T. Bryan in a statement. “But knowing that repetitive TBIs may make patients even more vulnerable provides new insight for attending to military personnel over the long-term, particularly when they are experiencing added emotional distress in their lives. ”

When Bryan and his team looked at the same issue over just the previous year, they found a similar, if somewhat less dramatic, trend: about 12% of veterans who had sustained multiple TBIs had suicidal thoughts, vs. only about 3.4% of those who had had only one. Again, none of the people without head injuries had suicidal thoughts. These results suggest that the pattern remains similar, but the risk for suicidal thoughts in general seems to decrease, the further away one is from the war.

The team, from the National Center for Veterans Studies at the University of Utah, hopes that their findings will help the medical and mental health communities evaluate who’s at greatest risk for suicidal thoughts and behaviors. They also want to understand why some people only think about suicide, while others actually act on it.

“Being aware of the number of a patient's head injuries and the interrelation with depression and other psychological symptoms may help us better understand, and thus moderate, the risk of suicide over time,” Bryan says. “Ultimately, we would like to know why people do not kill themselves. Despite facing similar issues and circumstances, some people recover. Understanding that is the real goal.”

The study is published in JAMA Psychiatry.