(CNN)Marty Smith, a prolific motocross rider known for winning 18 career national motocross titles, has died.
He was 63.
Smith and his wife Nancy were killed in a dune buggy accident in the California desert on Monday, close friend Lee Ramage confirmed to CNN.
Ramage and wife Tammi were in the buggy with the Smiths, riding through the Imperial Sand Dunes in California, when the vehicle rolled over. Lee Ramage and Tammi were unhurt.
They stayed with the Smiths in their last hour while the first responders arrived, he wrote in a Facebook post.
"He was my best friend and I did all I could to save them," Ramage told CNN over the phone. "It breaks my heart to know I couldn't save them."
Smith came into prominence during the 1970s, winning three national titles -- the 1974 and 1975 AMA 125cc and the 1977 AMA 500cc motocross championships. After retiring from racing in 1981, Smith was inducted into the American Motorcyclist Association Motorcycle Hall of Fame in 2000, according to the AMA.
Many posted their condolences.
Scott Champion, a current motocross racer, wrote on Instagram that he was shocked to hear the news.
"I'll never forget Marty approaching me in 2013 & believing in me when not many did," he said. "Working with Marty in 2014 & 2015 were some of the best times of my life that I will never forget! You will be missed by many my friend!"
JT Racing, a motocross clothing brand, also paid tribute.
"We are devastated by the passing today of motocross legend, Marty Smith and his wife Nancy. Our thoughts and prayers are with his family, friends and the entire motocross community," the company wrote.
Marty and Nancy are survived by three children and grandchildren, according to Ramage.