Opening arguments in the murder trial of Marine Corps veteran Eddy Ray Routh, the man accused of killing former Navy SEAL Chris Kyle, revealed Wednesday that Kyle had sent his friend, Chad Littlefield, a text message that read, "This dude is straight-up nuts," during their drive to the gun range on Feb. 2, 2013, where he was killed.

Washington Post reports Routh, the troubled Marine Corps veteran, had been sitting behind Littlefield in Kyle's Ford F-350 pickup on the way to the Rough Creek Lodge and Resort gun range in Iredell, Texas that day.

Littlefield replied to Kyle's text message with concern.

"He's right behind me. Watch my six," Littlefield wrote, referring to a military phrase that means "watch my back."

A short time later, Routh allegedly shot and killed Kyle and Littlefield at the gun range.

Routh's defense attorney, Tim Moore, introduced Kyle's text message to the court. Kyle had wanted to help Routh, who allegedly suffers from Post-Traumatic Stress Disorder (PTSD), at the request of Routh's mother. He and Littlefield had taken Routh to the resort to befriend him and engage in some "outdoor therapy."

Moore told the court that Routh had been diagnosed with paranoia, schizophrenia, psychosis and post-traumatic stress. He had undergone in-patient psychiatric treatment at two different mental health facilities in Dallas, but was released on Jan. 25, 2013 despite a determination on Jan. 19 that he could pose a danger to others, according to Moore.

The defense will argue that Routh is not guilty by reason of insanity.

Erath County District Attorney Alan Nash said that Kyle was shot five times, including once in the side of the head with a .45-caliber pistol. Littlefield was shot four times, once in the head, with Kyle's 9mm Sig Sauer pistol.

"Does mental illness take away the ability to know right from wrong?" Nash asked the jury. "I ask you to consider that."

The first witness called Wednesday was Taya Kyle, the Navy SEAL's widow, and she testified that she knew something was wrong when Chris spoke to her by phone from the Texas gun range shortly before he was killed.

Pausing to wipe away tears, Taya told the court, "I'm not nervous. I'm just emotional."

Her husband had never met Routh before Feb. 2, the day that they visited the Rough Creek Lodge gun range, she testified.

Taya said that she worried when her husband didn't respond to a subsequent text message, and she learned he had been killed when a police officer arrived at her house.

Chris, who is known as the deadliest sniper in American military history, retired from the military in 2009. He worked with veterans who, like himself, had a difficult time adjusting to civilian life. He regularly brought veterans to gun ranges.

The 40-year-old mother of two said that her husband believed that a week spent outdoors was more beneficial than a week in a hospital.

Following her testimony, the prosecutor showed crime scene photos, including one of Chris's dead body, during which Taya was seen shaking her head.

The Routh murder trial has received worldwide media attention as the popular "American Sniper" movie, based on a book by Chris and directed by Clint Eastwood, generates close to $300 million in ticket sales.

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