The legal saga of Eddie Ray Routh — convicted last week for killing “American Sniper” Chris Kyle and a friend — is not quite over; the former Marine’s lawyers say they have filed a motion for a new trial and notice of appeal to Ray’s life-without-parole sentence.

Routh faced a death penalty-eligible charge following the Feb. 2, 2013, shooting deaths of the former Navy SEAL Kyle, 38, and Chad Littlefield, 35. While it’s not entirely clear why prosecutors declined to seek the death penalty, the case demonstrates how a series of complex factors impact such a decision — even in a state like Texas, which has one of the largest death row populations in the country.

In this 2008 photo, Eddie Ray Routh poses with an AK-47 at Camp Fallujah in Iraq. (Courtesy of Corey Smalley via AP)

In October, Erath County District Attorney Alan Nash filed paperwork indicating he would seek a life-without-parole sentence for Routh instead of the death penalty, but he didn’t spell out why. Nash has not made any public statements explaining his rationale, nor has his office responded to multiple requests for comment about that decision.

[For Chris Kyle’s killer, Eddie Ray Routh, life in prison may make jail an asylum]

A death penalty conviction would have been unusual for Erath, though; the small, rural county of roughly 40,000 residents has never sentenced someone to die since the Supreme Court reinstated the death penalty in 1976.

In fact, fewer than half of the 254 counties in Texas — 120 to be precise — have sentenced a person to death since the high court’s decision. Half of the state’s death penalty convictions have taken place in just a handful of big counties: Dallas, Harris, Bexar and Tarrant.

“It’s an interesting feature of the death penalty in the United States,” Columbia University law professor James Liebman told The Post. “In fact, the death penalty is a minority institution. If you add up the populations of the counties that use the death penalty, it actually now is less than a quarter of the United States that really uses the death penalty. That’s even true in states that have the death penalty — many counties don’t use it.”

(Courtesy of James Liebman)

Death penalty use varies greatly across Texas, and for a number of reasons.

More populous counties — including the four that have sentenced half of the state’s death row inmates — record more violent crimes than smaller counties. There is also vast discretion on the use of capital punishment within the entire criminal justice system, said Jim Marcus, a clinical law professor at the University of Texas at Austin and co-director of the Capital Punishment Clinic.

[How the death penalty continued its slow, steady decline in 2014]

District attorneys are elected officials in Texas, Marcus noted, and some are known to seek the death penalty more frequently than others, whether it be due to personality or political pressures.

Marcus declined to discuss Routh’s prosecution. But speaking generally, he noted that money can play a role in capital murder cases. Death penalty trials are “hugely expensive” for a number of reasons, including the long appeals and ligation processes, Marcus said.

“They can be much more expensive, and if the prosecutor is looking at his or her annual budget, they might not have a couple thousand dollars to fund it, regardless of whether they think it’s the right punishment,” he said.

Counties “have a budget for criminal justice that they have to make go for the entire year, and the typical capital case is usually five, six, seven times more expensive than any other case,” said Liebman, the Columbia law professor.

Tara Kyle, the widow of slain Navy SEAL sniper Chris Kyle, cries on the witness stand on Feb. 11. (Pool photo by Tom Fox via European Pressphoto Agency)

Some small-county attorneys and judges launched the Regional Public Defenders for Capital Cases several years ago to help counties that can’t afford the cost burden of capital murder cases — including paying for public defenders. Erath County does not participate in the program, which is funded by fees from the participating counties and some state money.

“When we talk with prosecutors from around the state, it seems like a couple of things go into their decision,” said Jack Stoffregen, the program’s chief public defender. “It’s whether or not they feel fairly confident that a jury would return a sentence of death, and the other is cost.”

[Supreme Court stays execution of Texas inmate on death row for 30 years]

In Texas, the offense of capital murder is eligible for the death penalty. If a prosecutor opts against seeking that punishment, a guilty verdict triggers an automatic life-without-parole sentence, a change made in 2005. But if a prosecutor seeks the death penalty, a guilty verdict kicks off a second trial phase in which jurors consider mitigating circumstances in weighing whether a defendant should be sent to death row.

That means more time, more money spent on litigation — and more uncertainty.

“You know if you seek the death penalty, you’ve added years and millions of dollars to the proceedings, and you might not actually get the death penalty,” said former military prosecutor Chris Jenks, now an assistant professor at Southern Methodist University’s Dedman School of Law. “You really only want to take on the delay and the cost if you’re reasonably confident that you’re going to achieve that outcome; otherwise save yourself and the taxpayers years and millions of dollars.”

For Routh’s prosecutors, “there was at least a risk,” Jenks said. “We can debate about how great the risk was — [but] there was certainly a chance that he was not going to receive the death penalty.”

Routh’s defense lawyers argued that their client should be found not guilty by reason of insanity and pointed to past diagnosis of mental illnesses. His history of military service may have also given prosecutors pause in deciding whether to seek the death penalty, thinking jurors would be hesitant to send a troubled veteran to death row.

Routh is currently being held in the Erath County Jail.

[Marcus Luttrell, Navy SEAL friend of Chris Kyle, warns killer Eddie Ray Routh following verdict]

“It struck me as a really good decision by the prosecutor to not seek the death penalty simply because I think, to me anyway, the defendant, Mr. Routh, was somewhat of a sympathetic character, just based on what I read,” Stoffregen said. “And it would have made the prosecutor’s burden just a little bit heavier.”

Judy Littlefield is shown photos of her son, Chad, by a prosecutor during the trial. (Pool photo by Tom Fox via Dallas Morning News/AP)

Prosecutors also consider the wishes of victims’ families, Jenks said, although that’s not the deciding factor. Often, families privately convey such wishes to prosecutors.

After Ray was found guilty and sentenced to life in prison without parole, Chad Littlefield’s mother told the “Today” show “we feel justice was served.”

Death penalty trials are complicated affairs even before they begin. Jury selection can drag on, particularly in high-profile cases. Jurors are selected one by one, with each member of the jury pool questioned by attorneys, Marcus said. “It takes weeks to pick a capital murder jury,” he said. “You can pick a murder jury in an afternoon.”

Take the Boston Marathon bombing trial; jury selection began Jan. 5 with 1,373 people in the pool. It finally ended Tuesday with 18 people. Jurors who serve on such juries must be willing to vote for a death sentence.

There’s also a finality that takes place with non-death-penalty cases, said Kathryn M. Kase, executive director of the Texas Defender Service. If Routh’s conviction is upheld after an appeal, “then it’s over,” Kase said. “He has no more right to court-appointed counsel. That is in contrast to death penalty cases, where appeals go on and on and on, and the state must provide counsel at government expense.”

Routh’s motion is still pending, and no date has been set to hear it.

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