On Feb. 2, 2013, nearly four years after Kyle quit the military, he and his friend, Chad Littlefield, were allegedly shot and killed on a Texas gun range while helping former Marine Eddie Ray Routh, then 25, cope with his PTSD. Routh's trial is set to begin Feb. 11.
"My client will plead not guilty by reason of insanity," Routh's attorney, J. Warren St. John, tells PEOPLE. Prosecutors for the State of Texas have said they are not seeking the death penalty in the case.
One growing issue: With the film's success, finding an impartial jury may be difficult.
"The film will be an issue," St. John says. "I think any case with significant publicity has an issue with picking a jury. I've had them in the past, and anything that has significant national attention makes it hard to pick a jury."
St. John is unable to comment on any specifics of the case because of a court-mandated protective order preventing many involved from speaking to the media. (Citing the "unusually emotional nature of the issues involved in the case" and the "extensive local and national media coverage this case has already generated," a judge in 2013 issued the gag order preventing attorneys and family members and others from talking to the media.)
Routh's attorneys filed a motion in 2013 to change the location of the trial. "That was denied by the trial court, so now, you can't move the trial," St. John says. Routh has been in jail since the killings.
"No one wants to hurry up and do this, and then have it done wrong," says a court insider. "Everyone is taking the time to do it right."
Movie May Pose a ChallengeHouston criminal defense attorney George Parnham, who represented Andrea Yates (who was convicted of drowning her five children, then on appeal in 2006 found not guilty by reason of insanity), agrees that the movie poses challenges for Routh's trial.
"It's going to be very difficult for him to get a fair trial, not only because of the movie, but because of the media surrounding the movie," he tells PEOPLE. "Mr. Kyle is a hero in many people's eyes. Due to the fact that this movie has gained intense public attention, it's doubtful that a fair jury can be selected anywhere."
Parnham believes Routh's legal team should contest the gag order. "The only thing people will know about him is what they see in the movie," he says.
To win, Parnham says, Routh's lawyers will need to focus on the effects of PTSD and how "irrational" the shooting was.
"Kyle was reaching out to help this disturbed man, and ends up as a Good Samaritan, dead. That doesn't make sense," says the attorney. "This is an absolutely irrational act, and that has to be driven home to the general public."
With reporting by STEVE HELLING