The first commanding officer of SEAL Team Six, Richard Marcinko, passed away at the age of 81 on Saturday night.

His son, Matt Marcinko, and the Navy SEAL Museum announced his passing via social media on Sunday.

“Last night, Christmas evening, we lost a hero, who’s also known as The Rogue Warrior, the retired Navy SEAL commander AND the creator of SEAL Team Six, my father, Richard Marcinko,” wrote Matt Marcinko in a tweet. “His legacy will live forever. The man has died a true legend. Rest In Peace Dad. I love you forever”

“The Museum is very saddened to learn of the passing of Richard “Dick” Marcinko,” wrote the Navy SEAL Museum in a Facebook post. “‘Demo Dick’ was a retired U.S. Navy SEAL commander and Vietnam War veteran. He was the first commanding officer of SEAL Team SIX.”

“Marcinko was born Nov. 21, 1940, in Lansford, Pennsylvania, and enlisted in the U.S. Marines before being rejected for his lack of a high school diploma,” Fox News reported. He later earned a bachelor’s in international relations from the Navy Post Graduate School, and a masters in political science from Auburn University, according to navyseals.com. In 1958, he enlisted in the U.S. Navy and climbed the ranks to become a commander.

In January of 1967, Marcinko was deployed to Vietnam as part of the 2nd Platoon, SEAL Team Two, according to the Navy SEAL Museum. On May 18, 1967, Marcinko headed his men in the assault on Ilo Ilo Hon, which “became known as the Navy’s most successful SEAL operation in the Mekong Delta.” He and his men killed many Viet Cong and destroyed six of the enemy’s sampans.

Marcinko caused such disruption to the North Vietnamese Army that it placed a bounty on him, “payable to anyone who could capture and kill him. Marcinko was never caught,” per the Navy SEAL Museum.

He returned to the states before being deployed to Vietnam again with SEAL Team Two, where he took part in the Tet Offensive. “During the Tet Offensive, Marcinko ordered his platoon to assist U.S. Army Special Forces at Châu Đốc,” per the Navy SEAL Museum. “What began as an urban street battle evolved into an intense rescue mission of American nurses and a schoolteacher trapped in the city’s church and hospital.”

Over the course of two terms in Vietnam, Marcinko earned “the Silver Star, four bronze stars with combat V, two Navy Commendation Medals, and the Vietnamese Cross of Gallantry with Silver Star,” per navyseals.com.

Years later in 1979, Marcinko and another Navy representative served on a task force called the Terrorist Action Team (TAT), which was slated with devising a plan to rescue Americans during the Iran Hostage Crisis. The task force unsuccessfully carried out the mission Operation Eagle Claw, which prompted the Navy to establish a “full-time dedicated counter-terrorist team.” Marcinko was chosen to design and develop the team.

His project turned into SEAL Team Six. “At the time, the Navy had only two SEAL Teams. Marcinko named the unit ‘SEAL Team SIX’ in order to fool other nations, notably the Soviet Union, into believing that the United States had at least three other SEAL Teams that they were unaware of,” according to the Navy SEAL Museum.

SEAL Team Six developed into “the Navy’s premier counter-terrorist and hostage rescue unit,” and Marcinko served as its commander from August 1980 – July 1983. 

He went on to publish his autobiography entitled Rogue Warrior in 1993, which became a New York Times bestseller, according to Amazon.

Eric Olson, a retired SEAL admiral, “who commanded U.S. Special Operations Command from July, 2007 to August, 2011,” spoke with the Navy Times on Marcinko’s death.

“The SEALs who knew Dick Marcinko will remember him as imaginative and bold, a warrior at heart,” Olsen said. “He was a spirited rogue for sure, but we are better off for his unconventional service.”