STEPHENVILLE, Texas (AP) — After several days of evidence focused on the mental health of the former Marine accused in the fatal shootings of "American Sniper" author Chris Kyle and his friend, the trial of Eddie Ray Routh resumed Monday in Texas.

Criminal law experts say the case hinges on whether the defense can prove Routh, 27, was insane at the time and did not know the killings at a gun range constituted a crime. A Texas ranger who investigated the shootings and interviewed Routh testified Monday that Routh knew his actions were wrong.

Here is a look at key points in the case:

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WHO WAS KYLE?

Kyle served four tours in Iraq and made more than 300 kills as a sniper for SEAL Team 3, according to his own count. He earned two Silver Stars for valor. After leaving the military, he volunteered with veterans facing mental health problems, often taking them shooting. He took Routh to the shooting range at the request of the troubled veteran's mother.

The case has drawn intense interest, partly because of Kyle's memoir. An Oscar-nominated film based on the book has grossed more than $300 million in North America.

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PERSPECTIVES ON ROUTH

Family members say Routh suffered from post-traumatic stress disorder after serving in Iraq. Defense attorneys say Routh, who was taking anti-psychotic medication, was insane when Kyle and his friend, Chad Littlefield, took the former Marine to the shooting range to provide support and camaraderie. Routh, his lawyers say, believed the men planned to kill him.

Prosecutors say Routh was a troubled drug user who knew right from wrong, even with a history of mental illness.

Some of Routh's psychiatrists at Green Oaks Hospital, where he was admitted in September 2012 and in January 2013, are expected to testify.

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THE CRIME

On Feb. 2, 2013, Kyle, Littlefield and Routh drove to Rough Creek Lodge and Resort, which has a 1,000-yard shooting range. About 5 p.m., a resort employee discovered the bodies of Kyle and Littlefield on the ground amid scattered weapons; each had been shot several times. About 45 minutes later, authorities say Routh pulled up to his sister's Midlothian home in Kyle's truck and told her he had killed Kyle and Littlefield before driving away.

On Thursday, prosecutors presented a video in which officers spoke with Routh as he sat in Kyle's pickup. He refused to leave the vehicle and eventually sped off, with police in pursuit. The video shows a police vehicle ramming the pickup, which became disabled along the side of the road.

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TRIAL HIGHLIGHTS

Ranger Danny Briley, who interviewed Routh in the hours after the killings, was among those testifying Monday. Briley said Routh confessed to shooting the men.

"He stated that he knew it was wrong to kill them, that he wished he hadn't done it," Briley said under questioning by the prosecution.

Last week, a former deputy testified that he overheard Routh after he'd been taken into custody say he shot the men because they wouldn't talk to him as the three drove together to the shooting range.

Kyle's widow, Taya Kyle, testified about her husband's passion for helping veterans and gun safety. Sometimes choking up and wiping away tears, Kyle testified that her husband and Littlefield were close, and enjoyed spending time with veterans as they eased back into civilian life.

Prosecutors filed documents saying Routh smoked marijuana, drank excessively and had a history of killing small animals. On the day of the killings, Routh had been drinking and smoking marijuana and threatened his girlfriend with a knife, one of the documents says.

A Texas Ranger testified that authorities found marijuana, a near-empty bottle of whiskey and medication for schizophrenia while searching Routh's small wood-framed home after the shooting.

The testimony could show that Routh deliberately put himself into a violent state, said Park Dietz, a forensic psychiatrist who examined Andrea Yates, who was found not guilty in 2006 by reason of insanity in the drowning deaths of her five children.

"Voluntarily induced intoxication is not an excuse for the mentally ill," he said.

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WHAT'S AT STAKE

Jurors have three options: find Routh guilty of capital murder, find him not guilty or find him not guilty by reason of insanity. If convicted, Routh faces life in prison without parole. Prosecutors aren't seeking the death penalty. Even if he's acquitted, Routh could remain in custody. The Texas criminal code stipulates that in cases involving violent crimes where defendants are found not guilty by reason of insanity, the court can initiate civil proceedings to have them committed.

Copyright 2015 by The Associated Press. All Rights Reserved. This material may not be published, broadcast, rewritten or redistributed. (PHOTO: AP Photo/Routh Family, Erath County Sheriff's Office)