According to a police body camera video played in court Thursday, Eddie Ray Routh was sitting in Kyle's stolen truck when he told a detective: "I don't know if I'm going insane."
Routh also spoke of voodoo, hell and the apocalypse in the video. The defense, which is mounting an insanity case, focused on those remarks.
The prosecution asked Lt. Michael Smith of Lancaster, Texas, police whether he thought Routh was being metaphorical during their conversation. Smith said he could not tell.
A medical examiner with the Southwest Institute of Forensic Sciences was the last witness on the second day of the double-murder trial.
Dr. Jeffrey Barnard testified that two of the six bullets that hit Kyle would have been "rapidly fatal."
A prosecutor asked Barnard several times whether the other victim, Chad Littlefield, had wounds consistent with someone lying on the ground or in a kneeling position, but the doctor couldn't say.
The prosecution appeared to be attempting to establish that Littlefield and Kyle were defenseless when shot. Both men were shot multiple times at a Texas firing range in February 2013.
Victims had loaded guns
Kyle and Littlefield were wearing loaded handguns in their waistbands when they were found, an investigator testified Thursday.
Michael Adcock with the Texas Department of Public Safety said he also found shell casings, ammunition and several firearms on the ground when he arrived at the Rough Creek Lodge firing range outside Dallas.
The guns on Kyle and Littlefield's bodies were loaded, but two "old-style Western" revolvers were lying on the ground with all the bullets fired, Adcock testified. The slain men had fired these pistols at targets, the prosecutor has said.
The two men took Routh, a veteran with mental problems, to the firing range as a kind of therapy, lawyers have said.
After the killings, Adcock said, Routh drove Kyle's pickup to a Taco Bell off Interstate 35 and purchased two bean burritos, leaving a restaurant receipt in the driver's side door of the truck.
Another witness was Flavio Salazar of the police department in Lancaster.
Salazar said during a 30-minute standoff between Routh and police, he crawled to the truck and put "stop sticks" in front of the rear tires. Stop sticks have metal spikes that puncture tires.
Routh drove to I-35, stopped soon because of flat tires and was arrested, Salazar said.
The trial began Wednesday with a lawyer using Kyle's own words to bolster an insanity defense.
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.
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 9mm 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, directed by Clint Eastwood and starring Bradley Cooper, 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.
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.
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. Routh, an unemployed former Marine reportedly diagnosed with PTSD
, was gone, and so was Kyle's black Ford pickup truck, police said.
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.
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.
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.