Richard Marcinko, the creator of U.S. Navy SEAL Team 6, died on Saturday at the age of 81, according to his son, Matt Marcinko.

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

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

The Navy Seal Museum paid tribute to Marcinko’s death on Facebook.

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

Marcinko tried to enlist in the United States Marines, who rejected him due to a lack of a high school diploma. He successfully enlisted in the United States Navy in September 1958 as a radioman.

In 1961, he was accepted into the Underwater Demolition Team/Replacement (UDTR) training and graduated.

Marcinko 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, according to Concho Valley.

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 developed 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, per the news outlet.

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

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

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

SEAL Team 6’s most high-profile mission was the raid that killed Osama Bin Laden in 2011.

Marcinko was born on November 21, 1940, in Lansford, Pennsylvania. After retiring from the US Navy, he became an author, radio talk show host, military consultant, and motivational speaker.