A wall separated prisoners of war Joe Crecca and John McCain.
The two fighter pilots, captured by the North Vietnamese military in the late 1960s, were in solitary confinement in Hoa Lo Prison where American servicemen were imprisoned and tortured during the Vietnam War.
Crecca said his and McCain's cells were next to one another. They communicated by tapping on the wall.
"There's a wall between us, but there's an emotional bond between us that can't be broken. It's like steel," Crecca, now 77, told CNN affiliate KCPQ-TV. He was captured 11 months before McCain.
McCain, who died Saturday at age 81, spent 5 1/2 years as a prisoner of war at Hoa Lo, known to most Americans as the "Hanoi Hilton."
His Skyhawk jet was shot down over North Vietnam in 1967. He parachuted into a lake near Hanoi, breaking both arms and a leg, and was captured by communist soldiers. In captivity, McCain was tortured and beaten, an experience that left him with lifelong injuries, including severely restricted movement of his arms.
Like other American POWs, McCain and Crecca used a complex communication system known as tap code -- tapping out messages on the wall of their cells -- to communicate with each other.
"We would pass humorous things through the wall," Crecca told KCPQ. "You knew you succeeded when you could hear the guy on the other side laughing."
Crecca said McCain kept his spirits up, even through torture and the risk of "those communists re-breaking his arms."
During his time as a POW, McCain refused a preferential release offer, made because his father was an admiral. He turned down the offer until his comrades could also go home.
"He clearly was a man of extreme courage and of loyalty to the United States of America," Crecca said. "To his family, to his God, and to his country."
Decades later when McCain ran for president in 2008, Crecca and his wife, Joan, were there to support him during a fundraising trip in Seattle, Washington.
"He remembered the time when we were tapping through the wall to each other. And of all things, what he says to Joan is 'Joe was a great resister,'" Crecca said, surprised by his fellow veteran's remarks. "He endured much more than I did."
McCain rarely talked about what he went through in Vietnam, according to an October 2017 YouTube video commemorating the 50th anniversary of his capture.
McCain returned to visit Hoa Lo in April 2000. A photo from the visit is on display at the prison, which has been transformed into a museum.
"I put the Vietnam War behind me a long time ago," McCain said at the time. "But I harbor no anger nor rancor. I'm a better man for my experience, and I'm grateful for having the opportunity of serving."