Nearly 1 in 5 U.S. soldiers had a common mental illness, such as depression, panic disorder or ADHD, before enlisting in the Army, according to a new study that raises questions about the military’s assessment and screening of recruits.

More than 8 percent of soldiers had thought about killing themselves and 1.1 percent had a past suicide attempt, researchers found from confidential surveys and interviews with 5,428 soldiers at Army installations across the country.

The findings, published online Monday in two papers in JAMA Psychiatry, point to a weakness in the recruiting process, experts said. Applicants are asked about their psychiatric histories, and those with certain disorders or past suicide attempts are generally barred from service.

"The question becomes, ‘How did these guys get in the Army?’ " said Ronald Kessler, a Harvard University sociologist who led one of the studies.

A third study looked at the increased suicide rate among soldiers from 2004 to 2009. The study, which tracked nearly 1 million soldiers, found that those who had been deployed to Afghanistan or Iraq had an increased rate of suicide.

But it also found that the suicide rate among soldiers who had never deployed also rose steadily during that time. The study did not explain the cause.

The Pentagon did not make officials available Monday to discuss the studies.

The three studies are the first from a massive research initiative started in 2009 by the Army and the National Institutes of Mental Health in response to the surge in suicides.

In 2011, a representative sample of soldiers was extensively questioned and assessed for a history of eight common psychiatric disorders.

Traditionally, the Army has been psychologically healthier than the rest of society because of screening, fitness standards and access to health care. Soldiers committed suicide at about half the rate of civilians with similar demographics.

But researchers found that soldiers they interviewed had joined the Army with significantly higher rates of post-traumatic stress disorder, panic disorder and attention deficit and hyperactivity disorder than those in the general population.

Most notably, more than 8 percent of soldiers entered the Army with intermittent explosive disorder, characterized by uncontrolled attacks of anger. It was the most common disorder in the study, with a pre-enlistment prevalence nearly six times the civilian rate.

"The kind of people who join the Army are not typical people," Kessler said. "They have a lot more acting-out kind of mental disorders. They get into fights more. They’re more aggressive."

The researchers found that despite screening, pre-enlistment rates of depression, anxiety, bipolar disorder and substance abuse were on par with civilian rates.

Rates of suicidal ideation, planning and attempts were lower than in the general population but still significant, given the military’s practice of excluding recruits with a known suicidal history.

During their military service, the soldiers’ rates of most psychiatric disorders climbed well past civilian levels, several times the rate for some disorders.

A quarter of soldiers were deemed to be suffering from a mental illness — almost 5 percent with depression, nearly 6 percent with anxiety disorder and nearly 9 percent with PTSD. The percentage of soldiers who had attempted suicide rose from 1.1 percent to 2.4 percent.

Matthew Nock, a Harvard University psychologist who led the study on suicide, said more than 30 percent of suicide attempts that occurred after enlistment would have been prevented if the Army had excluded recruits with pre-existing mental health conditions.

Nock said he believed the Army should improve its screening of recruits, not to exclude them but to provide treatment to those who acknowledge a history of mental illness.

Screening out mentally ill recruits is not as simple as it sounds because the military largely has to rely on applicants to disclose their mental health histories.

"People who want to come into the Army are no fools," said Dr. Elspeth Ritchie, a former chief psychiatrist in the Army.

"They know if you say you had a past suicide attempt, you’re probably not going to get in."

Dr. Eric Schoomaker, who served as surgeon general of the Army until 2012, said more stringent screening "would just lead to driving the problems further underground."

In addition, the military would not meet its recruiting targets if it were able to identify and exclude everybody with a history of mental health problems, experts said.

During the peak years of war, as the military was struggling to fill its ranks, some recruiters were known to discourage applicants from disclosing such problems.