STEPHENVILLE, Tex. — On the way to the outdoor resort where former Navy SEAL Chris Kyle was killed, he sent a text message to the friend sitting next to him about the third man in the truck: “This dude is straight-up nuts.”

Kyle, famous for his depiction in the recent movie “American Sniper,” sent the message to Chad Littlefield, his friend and weight-lifting partner, about the troubled Marine Corps veteran sitting behind Littlefield in Kyle’s Ford F-350 pickup. That man, Eddie Ray Routh, 27, is accused of shooting and killing Kyle and Littlefield a short time later at the Rough Creek Lodge and Resort, an 11,000-acre facility that included a gun range where Kyle had brought Routh in an attempt to help him.

Opening arguments in Routh’s murder trial began Wednesday here in Erath County, near where Kyle and Littlefield were found dead on Feb. 2, 2013. The case has received international media attention as the “American Sniper” movie, directed by Clint Eastwood and based on a book by Kyle, nears $300 million in ticket sales.

[RELATED: Taya Kyle, wife of slain Navy SEAL Chris Kyle, delivers tearful testimony]

Kyle’s text message about Routh was introduced by one of his defense attorneys, Tim Moore. Kyle and Littlefield had taken Routh to the resort at the request of Routh’s mother, who told them the veteran was struggling with mental illness. Littlefield responded to Kyle’s text message with one of his own indicating he was concerned, too.

“He’s right behind me. Watch my six,” Littlefield wrote, using a military phrase meaning “watch my back.”

The case promises to focus heavily on Routh’s military background and history of mental illness. Moore told the court Wednesday that Routh had been diagnosed with a variety of conditions, including psychosis, paranoia, schizophrenia and post-traumatic stress. The defense will argue that Routh is not guilty by reason of insanity and didn’t know that what he was doing was wrong.

Routh was “in the grip of a psychosis” when he killed Kyle and Littlefield, Moore said. He said his client had undergone psychiatric treatment at both the Green Oaks Hospital and the Department of Veterans Affairs hospital in Dallas. It was determined on Jan. 19, 2013 — just a couple weeks before the shooting — that he could pose a danger to others, but the VA hospital released him on Jan. 25 with anti-psychotic medicine, Moore said.

Erath County District Attorney Alan Nash told the jury that Kyle was shot five times in the back and side and once in the side of the head with a powerful .45-caliber pistol. Littlefield was shot four times in the back, once in the hand and once in the head with Kyle’s 9mm Sig Sauer pistol, which had a Navy anchor engraved on it and was later found, reloaded, with Routh when he was arrested, Nash said.

“There was no saving these men,” Nash said of Kyle and Littlefield. “Their injuries were too bad. They were beyond saving.”

Nash told the court that while Routh was troubled, he knew what he was doing was wrong when he killed Kyle and Littlefield.

“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 SEAL’s widow. They had an 8-year-old son and a 6-year-old daughter together at the time of his death. Her testimony is expected to continue through Wednesday afternoon.

A frantic 911 call details the aftermath of the murder of "American Sniper," Chris Kyle. The call comes from the suspect's sister and brother-in-law. Eddie Ray Routh is accused of killing Chris Kyle and his friend, Chad Littlefield. (Jorge Ribas, Kate M. Tobey and Julio C. Negron/The Washington Post)

This post was updated to correct the spelling of Alan Nash’s name.