Iraq War veteran Eddie Ray Routh is preparing to stand trial, charged with capital murder in the shootings of "American Sniper" author and former Navy SEAL Chris Kyle and his friend Chad Littlefield. Routh's post-traumatic stress disorder brought the men together, as Kyle and Littlefield sought to help the ex-Marine. Routh's parents think that diagnosis should carry some weight with the jury. Here are some questions and answers about the proceeding, and how PTSD might play a role.

Q: Did Eddie Ray Routh and Chris Kyle know each other?

A: No. Mother Jodi Routh has told numerous news outlets that she ran into Kyle at the school where she serves as an aide. She had heard about Kyle's work with wounded veterans and asked if he might be willing to help her son, who was struggling with PTSD and alcohol abuse. Kyle and Littlefield went to Routh's home on Feb. 2, 2013, to take him to a shooting range, where the killings occurred. It appears to have been the first time they'd met.

Q: What is Routh's defense strategy?

A: Since Routh admitted the slayings to his sister and brother-in-law, and in an interview with the Texas Rangers, the trial will revolve around his state of mind at the time. Routh's attorneys have filed a notice of intent to pursue an insanity defense. Prosecutors have chosen not to seek the death penalty if Routh is convicted, but will ask that he be given life without the possibility of parole. Defense attorneys have expressed concern about whether Routh can receive a fair trial, since the proceeding, set to begin Feb. 11, will come as the Oscar-nominated movie "American Sniper," based on Kyle's memoir, is filling theaters nationwide and even in Erath County, Texas, where trial will be held.

Q: What is PTSD?

A: According to the American Psychiatric Association, PTSD is characterized by "clinically significant distress or impairment in the individual's social interactions, capacity to work or other important areas of functioning." Paula Schnurr, acting executive director of the National Center for PTSD, says the individual need not have experienced the trauma himself. "PTSD can also occur when people have witnessed a horrific event occurring to others or learning about some types of horrific events that may happen to a loved one, such as losing a loved one to murder or suicide," said Schnurr, whose agency is part of the U.S. Department of Veterans Affairs. Symptoms include recurrent dreams or flashbacks, changes in mood, avoidance and hypervigilance. Anger and irritability are also in the profile, though not all people experience those symptoms, says Schnurr. "We know from research that individuals with PTSD have an increased likelihood of engaging in aggressive or violent behavior," she said. "There is a statistical association between PTSD and violence, both domestic violence and violence against others. But when I say violence I mean aggression or violence and that may include threats and not acts, so it's a broad category." An estimated 15 percent of Iraq and Afghanistan veterans have PTSD.

Q: How has the PTSD defense fared in the criminal courts?

A: The record has been decidedly mixed. The authors of a 2012 article in The Journal of the American Academy of Psychiatry and the Law concluded that success hinges largely on how well the expert testimony links the symptoms and crime. "Appellate courts have found criminal defenses based on PTSD to be viable and compelling when a clear and direct connection between the defendant's PTSD symptoms and the criminal incident was found by the expert," wrote the authors of "PTSD as a Criminal Defense: A Review of Case Law." ''The PTSD phenomena that appellate courts have found to be most relevant to criminal defenses include dissociations, hyperarousal symptoms, hypervigilance symptoms, and the overestimation of danger." Some jurisdictions have recognized PTSD "as a valid basis for insanity, unconsciousness, and self-defense," the authors found. However, Georgia recently executed Vietnam veteran Andrew Brannan in the 1998 murder of a deputy sheriff, despite arguments from his attorneys that he had PTSD and was 100 percent mentally disabled.

Q: Much has been written about "American Sniper" author Chris Kyle. But who was the other victim, Chad Littlefield?

A: Littlefield, 35, had a wife and young daughter. According to a June 2013 New Yorker Magazine story, the two men met on the sidelines of a youth soccer game and became friends. They lived in the same neighborhood, and hunted and worked out together. Although he had never been in the military himself, Littlefield - facilities and logistics manager with a lab in DeSoto, Texas - also volunteered his time to work with veterans. By the time of that fateful outing with Routh, he had accompanied Kyle "on similar trips dozens of times," according to an April 2013 article in D Magazine. Routh's trial is set to begin on what would have been Littlefield's 38th birthday.