(USA Today) -- In the immediate moments after a freak skiing accident shredded Shawn Johnson's left knee two years ago, her first thought was: "What if I can never do gymnastics again?"
Fueled by that what-if, Johnson began an improbable comeback attempt last year.
Sunday, her Olympic journey came to end as the 20-year-old from Des Moines announced her retirement from competitive gymnastics. Johnson, one of the most popular U.S. gymnasts of all time, is also one of the most decorated. Her career includes an Olympic gold medal and three silver at the 2008 Beijing Games and three gold at world championships.
"This is a really hard moment," Johnson said Sunday. "I still have the heart, drive and desire to compete. Unfortunately my road has been cut a little short."
Johnson reached her decision Friday after discussions with her coach Liang Chow and national team coordinator Martha Karolyi. She knew then that her left knee, which was surgically repaired after she tore her ACL and MCL, was not able to sustain the demands of continued training.
"It's been a constant fight. It came down to there wasn't enough time left to heal," Johnson said. "I couldn't push myself any further. My knee physically wouldn't let me do anymore."
With nationals in St. Louis beginning on Thursday, followed by Olympic trials three weeks later in San Jose and Olympic team selection on July 1, Johnson said she didn't want to be a distraction to those in contention.
"I thought it was the right time," she said. "I wanted as much attention on the gymnasts making the team."
Earning a spot on the five-member team would have been difficult given the depth of U.S. talent.
"We have such talented girls and so many of them, it's going to be very cut-throat," Johnson said. "If I was at 100 percent competing, it would have taken everything I had and luck. It's one of the greatest groups I've seen."
Of the 2008 Olympic gymnasts hoping to make their second Games, Johnson became the latest departure in a dwindling pool. Fellow Beijing teammate Chellsie Memmel saw her Olympic hopes end less than two weeks ago when she failed to qualify for nationals. It appears only Nastia Liukin, the 2008 all-around Olympic gold medalist, has a shot at making the team as a specialist.
"Shawn will always hold a special place in gymnastics and my heart," Karolyi said in a statement. "She always showed the joy of doing gymnastics. Shawn was an excellent student in training and a fierce competitor in the arena. I will miss her, but I am confident that she will be very successful in any endeavor that she decides to pursue."
Johnson said she will start college in the fall of 2013 and is deciding between Vanderbilt and Stanford. "College has always been the one must," she said.
She will also be in London this summer on behalf of her sponsors. Though she said it will be difficult to sit in the stands and watch, she plans "to scream my head off" for her teammates.
Johnson, who won Dancing With the Stars in 2009, has been one of the more visible U.S. athletes in the run-up to the Olympics, given her high-profile national sponsorships.
"She represented that girl-next-door and that true spirit of America, showing people how much she enjoyed what she was able to experience. In the big moments on the biggest stages in the world, Shawn soaked it all in," said Steve Penny, president of USA Gymnastics.
Still dealing with the roiling emotions of her decision, Johnson isn't sure what tomorrow will bring, especially when it's 2:30 p.m., the time she would always head to the gym.
"It's hard to accept that the one thing you had your entire life is no longer there," she said. "I'm scared to death to know what the future holds. It's a road I never thought about. I'm excited as well, ready to start the new journey."