Updated: 7:36 p.m. EDT


Big Brown backed up his trainer’s boasts with an explosive finishing kick and won the Kentucky Derby on Saturday, a commanding victory turned somber by the fatal breakdown of the filly Eight Belles on horse racing’s biggest day.


The cheers for the winner’s 4 3/4-length victory were cut short when Eight Belles, the runner-up, collapsed while galloping out near the second turn. She broke her two front ankles and was euthanized on the track minutes later.


“When we passed the wire I stood up. She started galloping funny. I tried to pull her up. That’s when she went down,” said her distraught jockey, Gabriel Saez.


Eight Belles was attempting to become the fourth filly to win the Derby.


The unbeaten Big Brown’s start from the outside post did little to hamper his charge when the 20-horse field turned for home at Churchill Downs. Under the urging of jockey Kent Desormeaux, the 2-1 favorite cruised to an easy victory to become the seventh unbeaten Derby winner with his fourth consecutive win. The last one was Barbaro in 2006.


The breakdown brought back memories of the 2006 Preakness, where Barbaro shattered his right rear leg just after the start. The colt was euthanized months later, after developing laminitis from the catastrophic injuries.


The tragedy undoubtedly drained some of the joy from Big Brown’s victory, which sends him to the Preakness in two weeks as the only 3-year-old with a chance to become the first Triple Crown champion since Affirmed in 1978.


“We’re ready to roll,” Big Brown’s confident trainer Rick Dutrow Jr. said.


All week, Dutrow told the world he had the best horse in the field — and the big bay colt justified every accolade tossed his way.


“I can’t describe the feeling that all of us have right now,&#