The suspect accused of shooting dead former Navy SEAL sniper Chris Kyle and another man stole a vehicle from the scene and told his sister "I traded my soul for a new truck," investigators said.
Eddie Ray Routh was being held on $3 million bond on a capital murder charge for the double-slaying on Saturday.
The fact that Routh, 25, of Lancaster, Texas, served in the Marines from 2006 to 2010, including a tour in Iraq, put attention on the psychological wounds of war. The new evidence, which first came to light on Monday, suggests robbery could have formed part of the motive.
Kyle, author of the book "American Sniper," was credited with more than 150 kills as a sniper for U.S. forces in Iraq and had been working with former warriors recovering from physical and emotional injuries, sometimes inviting them to the shooting range.
Randy Fowler, an investigator with the Erath County Texas Sheriff's Department, wrote in the affidavit that Routh drove to his sister's home in Midlothian, about 50 miles from the gun range where the shooting took place, shortly after the incident.
Routh was driving what his sister, Laura Blevins, described as a "big dark or black Ford F-250 pickup that she had never seen before," Fowler said in the affidavit.
"It substantiated Routh's claim that he had murdered Chris Kyle and his friend, and he told the Blevinses that he had killed Kyle and that he had 'traded his soul for a new truck'," Fowler wrote.
Routh was later arrested at his home in Lancaster Texas, south of Dallas, after leading officials on a brief police chase while he was at the wheel of the truck, which has been identified as the truck that Kyle, Routh, and Kyle's friend Chad Littlefield drove in to the shooting range earlier that day.
Routh's sister advised him to turn himself in, but Routh said he wanted to get to Oklahoma to avoid Texas authorities, the affidavit said.
Kyle, a friend identified as Chad and "another vet" arrived at the shooting range about 3:15 p.m. Saturday, the affidavit said, an employee of the Rough Creek Lodge shooting range.
"At around 4:50 p.m., he (another employee) went to the shooting range and found Chris and Chad lying on the ground covered in blood," the report said of one employee.
Twice in recent months, Routh was taken to a mental hospital after behaving erratically, according to police reports from Dallas and his hometown of Lancaster.
Investigators have discussed Routh's troubled past, including brushes with the law in Lancaster and in Dallas.
Dallas television station KTVT quoted relatives of Routh saying they believed he was a victim of the government's inability to provide the mental health treatment he needed for the emotional problems he suffered in combat.
Police in Lancaster took Routh to a mental hospital last fall, after family members told officers he was threatening to kill himself during an argument with his father, KTVT reported.
Law enforcement officers have not said Routh specifically suffered from post traumatic stress, a severe anxiety disorder caused by witnessing or participating in traumatic events, but the killings renewed the focus on PTSD among veterans.
About 30 percent of returning Iraq and Afghanistan war veterans suffered from some form of post-traumatic stress, the Department of Veterans Affairs estimated in a report released last fall.
Kyle, who served four combat tours of duty in Iraq, won two Silver Stars and five Bronze Stars for bravery, according to his book, which covers his military service from 1999 to 2009.
(This story corrects name of sister to Laura Blevins, 6th paragraph)
(Editing by Daniel Trotta; Editing by Alden Bentley)