The two-week trial in Erath County, about 100 miles (160 kilometers) southwest of Dallas, attracted massive media attention across America, coinciding with the release of Clint Eastwood`s controversial movie based on Kyle`s experiences in Iraq.
Judy Littlefield, the mother of Chad Littlefield, made a brief statement late Tuesday after the verdict.
"We have waited two years for God to give us justice on behalf of our son, and God has proved to be faithful," she said.
"We are so thrilled that we have the verdict that we have."
Kyle, officially credited with killing 160 people during four military tours, was lionized in the blockbuster film starring Bradley Cooper as the soldier.
Routh`s attorneys pleaded insanity and said their client was gripped by psychosis. Himself a former Marine, Routh also said he suffered from post-traumatic stress disorder.
After the shootings on February 2, 2013, Routh took Kyle`s truck and was later arrested at the home of his sister.
He allegedly confessed to the killings after complaining that "people were sucking his soul and that he could smell the pigs."
During the trial, jurors heard how Kyle and Littlefield knew something was badly wrong minutes before Routh opened fire.
"This dude is straight up nuts," Kyle wrote to Littlefield in an alarmed text message.
"He`s right behind me, watch my six," replied his friend, using military jargon for "watch my back."
Jurors also heard emotional testimony from Kyle`s widow Taya, who sobbed as she recounted their life together.
Lawyers had questioned whether Routh could receive a fair trial in the small town, given the success of "American Sniper" and the widely held view of Kyle as a hero.
"American Sniper" has so far earned more than $320 million to become the highest grossing war film in history.
Critics claim it presents a simplistic, black-and-white view of the Iraq conflict, and glosses over Kyle`s references to Iraqis in his memoir as "savages."
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