The crash happened at about 1:50 a.m. Thursday morning along Santa Monica Avenue, according to San Diego police.Police said the man was riding his skateboard on his stomach and grabbed onto the undercarriage of a tractor trailer. The man was crushed when the truck turned, and police said the man rolled under the truck's wheels. Police said the skateboarder crushed his pelvis and has massive internal injuries.Several Ocean Beach residents told 10News the victim is 36-year-old Brian Conner, a homeless Army veteran. They said he lost his legs while serving in Bosnia in the 1990s.Photographer Jim Grant, who recently took a photo of Conner, said, "He's kind of hard to miss because he's on a skateboard and he doesn't have any legs and he pushes himself around on his knuckles."Grant and Conner's friends told 10News this is not the first time the double amputee has hitched a ride by grabbing onto a moving vehicle."I've seen him grab onto cars and kind of be pulled up a couple of times and I always thought, 'Whoa, that's pretty dangerous. That's kind of like 'Back to the Future' stuff,' and it's not too smart," said Grant.Grant was referring to a similar stunt in the 1980s movie performed by actor Michael J. Fox.A UC San Diego Medical Center spokeswoman told 10News Conner remains in critical condition."That's just very unfortunate that something like that had to happen," said Grant.Police did not say if Conner was under the influence of drugs or alcohol. They said the truck driver never saw the man on the skateboard.