Former Navy Seal Marcus Luttrell is shown here at the capital murder trial of Eddie Ray Routh at the Erath County Donald R. Jones Justice Center in Stephenville, Tex., on Feb. 24. (Michael Ainsworth/Dallas Morning News via AP)

Marcus Luttrell, a Navy SEAL veteran who befriended the late Chris Kyle and inspired the 2013 movie “Lone Survivor,” took to social media Tuesday night to respond to the guilty murder verdict of Kyle’s killer — and to share a warning.

In a post distributed on Facebook and Twitter, Luttrell said justice was served in the case and warned Routh, 27, to “Wait till the boys in TDC Find out you killed a TX hero.” It’s an apparent reference to the Texas Department of Criminal Justice, which handles prisoners.

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

Routh’s family has said he struggled with post-traumatic stress disorder when he killed Kyle, 38, who inspired the movie “American Sniper,” and his friend Chad Littlefield on Feb. 2, 2013. The legitimacy of that has been questioned by some veterans, who note that Routh never saw direct ground combat while serving in the Marine Corps.

[RELATED: New effort to get Chris Kyle the Medal of Honor faces uphill battle]

Luttrell was in court Tuesday to hear closing arguments in support of Kyle and the Littlefield families. He and Kyle were friends for years before the killings in 2013, according to Kyle’s “American Sniper” memoir.

Luttrell is best known for his involvement in Operation Red Wings, a June 2005 mission in Afghanistan in which he and his SEAL teammates were ambushed by dozens of Taliban fighters. Luttrell’s fellow SEALs — Michael P. Murphy, Danny Dietz and Matthew Axelson were all killed in intense fighting, and 16 more Special Operations troops died when their helicopter was shot down attempting to rescue Luttrell’s team.

Luttrell was the only one to make it out alive. He was protected by local villagers, and recovered by U.S. troops after they sent someone for help. He received the Navy Cross, second only to the Medal of Honor, for his actions in the operation. Murphy received the Medal of Honor posthumously, and Axelson and Dietz also received the Navy Cross.

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Police describe high-speed chase to capture Eddie Ray Routh

Portrait emerges of Chad Littlefield, friend killed alongside Chris Kyle

Taya Kyle, wife of slain Navy SEAL Chris Kyle, delivers tearful testimony

‘This dude is straight-up nuts’: Chris Kyle’s text message about Eddie Ray Routh