'Couldn't save them': Motocross legend and wife killed in freak accident

Motocross legend Marty Smith and wife Nancy have been killed in a dune buggy accident in the California desert.

The 63-year-old Marty Smith won 18 national motocross championships across an illustrious career.

A close friend who was with Marty and Nancy at the time told CNN they were killed after the buggy they were riding in rolled.

'THAT’S RUDE': Scott McLaughlin at centre of fresh Bathurst controversy

FAMILIAR FACES: Ricciardo's stunning admission about return to Red Bull

Lee Ramage said he and his wife were also riding in the buggy and stayed with the Smiths until first responders arrived.

“He was my best friend and I did all I could to save them," Ramage told CNN.

“It breaks my heart to know I couldn't save them.”

Marty Smith was reportedly pronounced dead at the scene while Nancy died later in hospital.

Smith was reportedly not wearing a safety harness.

“It’s with the heaviest, grieving heart that I have to tell you Marty and Nancy Smith were killed yesterday in a rollover accident in his dune buggy at the imperial sand dunes,” Rampage also wrote on Facebook.

“My wife, Tammi and I were in the buggy and were unhurt.

“We spent at least an hour trying to save their lives before first responders arrived.

“Marty took his last breath while I was holding his head, keeping his airway clear.

“Marty was my best friend and I tried everything in me to save him while Tammi tried to save Nancy.”

Smith won back-to-back AMA 125cc National Motocross Championships after turning pro at 17.

In 1977 he won the 500cc championship before retiring in 1981.

He was inducted into the AMA Motorcycle Museum Hall of Fame in 2000.

Motocross world in shock over Marty Smith’s death

Motocross star Scott Champion expressed his shock on Instagram.

“I'll never forget Marty approaching me in 2013 & believing in me when not many did,” he wrote on Tuesday.

“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!”

Clothing brand JT Racing also paid tribute.

“We are devastated by the passing today of motocross legend, Marty Smith and his wife Nancy,” the company wrote.

“Our thoughts and prayers are with his family, friends and the entire motocross community.”