Stephenville, Texas (CNN) The trial of the man accused of killing Chris Kyle, the protagonist in the top-selling book and hit movie "American Sniper," began Wednesday with a lawyer using Kyle's own words to bolster an insanity defense.

Eddie Ray Routh is accused of killing Kyle and Kyle's friend, Chad Littlefield, at a firing range at Rough Creek Lodge, about 90 miles southwest of Dallas on February 2, 2013.

In his opening statement, defense attorney Tim Moore read a text Kyle sent to Littlefield about Routh as the three men rode in Kyle's pickup truck to the range.

"About an hour and a half into the drive, Chris Kyle was sitting in the driver's seat and he texts Chad Littlefield sitting right next to him. He texts, 'This dude is straight-up nuts,'" Moore said. "Chad Littlefield texts Chris Kyle back, 'He's right behind me, watch my six [military lingo for 'watch my back.']. So while we don't know what the conversation was, we do know what Chris Kyle was thinking at the time he was in that truck."

Moore said Routh killed the two men because he suffered "a psychosis so severe that at that point in time he did not know what he was doing was wrong. ... He thought in his mind at that moment in time it was either him or them."

But Erath County District Attorney Alan Nash told the jury that Routh told investigators he used drugs and drank whiskey that morning. He admitted that he killed the two men and said he "knew what he was doing was wrong," the prosecutor said.

Prosecutor: Kyle unable to defend himself

Routh used two guns, Nash said.

Routh shot Kyle five times in the back and side and once in the side of the head, using a .45-caliber pistol, and shot Littlefield with a 9 mm pistol four times in the back, once in the hand, once in the face and once in the head, Nash said.

A different .45-caliber pistol with all its rounds fired was found on the ground near Kyle's head, Nash said. "(Kyle) had shot all the bullets in his gun when he was shot in the back," said Nash, referring to the fact that Kyle had just fired at a target.

The trial comes just weeks after the release of the film about Kyle, a former Navy SEAL who claimed to be the deadliest sniper in U.S. history with 160 confirmed kills in Iraq. The film has grossed more than $280 million, the most ever for a war movie, and the autobiography by the same name spent weeks on best-seller lists.

The first witness with Kyle's widow. Taya Kyle recounted the last conversation with her husband, on the phone.

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"I said are you OK?" she said. "He said 'yep.' And that's not common for him. I could tell something was up and he was just quiet ..."

On that day, Kyle took Littlefield and Routh, a troubled veteran he was trying to help, to the firing range.

Taya Kyle said her husband sounded irritated.

"Normally, going out there, especially a place like Rough Creek -- usually it's beautiful. He feels really good about helping somebody. He's making their day and he knows it," she testified. "Earlier, he thought that guy sounded really excited to go, so he thought he was doing a good thing."

She said their last conversation "was very short, and it wasn't short like, 'Hey, you are interrupting a good time.' It was short like, 'I wish I could say more but I'm not going to because there were people around.' "

Later she texted and he didn't reply, and she became worried, she testified.

A day at the range

Kyle had already risen to fame through his book when he died. He'd been doing charitable work to help former troops suffering from post-traumatic stress disorder.

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The range is a small, remote part of the sprawling 11,000-acre Rough Creek Lodge, and the men were isolated, authorities said.

Frank Alvarez, resident manager of Rough Creek Lodge, testified Kyle had "exclusive access to the range when he came out."

Kyle had said he was going to use the range for about 45 minutes, Alvarez testified, but "about 5 p.m. I got a radio call that said I had to go to the shooting range. Something had happened."

A hunting guide found Kyle, 38, and Littlefield, 35, Kyle's friend, motionless and called 911. The men were dead when officers arrived. Jurors were shown graphic photos of their bodies. Taya Kyle was in the courtroom at the time.

Routh's sister: 'He's all crazy'

Routh drove up in Kyle's truck at his sister's house 65 miles away, police said. She called 911, telling the operator he claimed to have killed two men.

"They went out to a shooting range. Like, he's all crazy," Routh's sister told authorities.

Routh got back into the truck and hit the road again, police said. Officers caught up with him that evening at his home in a Dallas suburb.

While talking with police, he jumped back into the truck and sped off again, police say. They gave chase and stopped him after spiking his tires. He did not struggle when they arrested him, police said.

Relatives of Routh, 27, and those close to him declined interview requests from CNN. Routh's attorney is making the case that his client is not guilty by reason of insanity.

Since July 24, 2013, when a judge filed a gag order in the case, nobody associated with Routh's trial has been permitted to speak to the media.

Before that order was issued, a reporter asked Capt. Jason Upshaw of the Erath County Sheriff's Office what could have driven Routh to the alleged murders.

"I don't know that we'll ever know," Upshaw said.

Routh served in the Marines from June 2006 to June 2010. His time in the military included a 2007 tour of duty in Iraq and a humanitarian mission to help the victims of the devastating 2010 earthquake in Haiti.

American Sniper

Kyle learned to shoot on hunting trips with his father, then went on to serve four combat tours in Iraq with the SEALs, though his official biography notes he also worked with Army and Marine units.

He received two Silver Stars and other commendations before leaving the Navy in 2009 after 160 confirmed kills, which he called a record for an American.

He said that while killing did not come easy at first, he knew it meant saving lives.

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"The first time, you're not even sure you can do it," he said in the interview. "But I'm not over there looking at these people as people. I'm not wondering if he has a family. I'm just trying to keep my guys safe."

Kyle's story and the movie made from it have triggered broad enthusiasm but also drawn critics and doubts about his accounts.

Correction: Earlier versions of this report incorrectly described the military background of Chad Littlefield, who was killed with Chris Kyle. Littlefield was not a veteran.