(The Hill) – Richard Marcinko, founding commander of U.S. Navy SEAL Team 6, died on Saturday at the age of 81, his son announced 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,” his son Matt Marcinko tweeted.  

“His legacy will live forever. The man has died a true legend. Rest In Peace Dad. I love you forever,” he continued.

Matt Marcinko told The New York Times that the cause of his father’s death was believed to be a heart attack.

The Navy Seal Museum, which is not affiliated with the Navy, acknowledged Marcinko’s death on Facebook.

“Dick Marcinko played a very unique part in SEAL history, leaving a legacy like no other,” the museum wrote in a Facebook post. “’Demo Dick’ is considered the United States’ premier counterterrorism operator. We send our deepest sympathies to his family, teammates, and friends.”

Marcinko enlisted in the Navy in 1958 and would work his way up to commander, according to Navyseals.com. He served two different tours in Vietnam, during which he earned the Silver Star, four Bronze Stars, two Navy Commendation Medals and the Vietnamese Cross of Gallantry with Silver Star.

Marcinko was later promoted to lieutenant commander and served in Cambodia before assuming command of Navy SEAL Team 2 in Vietnam, according to the museum’s Facebook post.

During the Iran hostage crisis in 1979, Marcinko was one of two Navy representatives on a Joint Chiefs of Staff task force known as the Terrorist Action Team (TAT), which was tasked with developing a plan to free American hostages. That team’s work culminated in Operation Eagle Claw in 1980, the failed attempt to rescue the hostages from the U.S. embassy in Tehran.

After that mission, Marcinko was asked to develop a team that would specialize in countering terrorist threats.

The Navy only had two SEAL Teams at the time, but Marcinko named the team “SEAL TEAM 6” to fool other nations into believing that the U.S. had at least three other such teams that were unknown.  

He gathered the team’s members from several existing SEAL teams and Underwater Demolition teams.

Marcinko commanded the team from August 1980 to July 1983, the museum noted, but SEAL Team 6 went on to become the Navy’s premier counterterrorist and hostage rescue unit.

The team’s most high-profile mission was the raid that killed Osama Bin Laden in 2011. The operation was later adapted into books and movies. 

Marcinko retired in 1989, and later became a best-selling author, motivational speaker and consultant, the Times noted.