LOUISVILLE, Ky. -- Big Brown backed up his trainer's boasts with an explosive finishing kick and won the Kentucky Derby on Saturday -- a day marred by the fatal breakdown of the filly Eight Belles.

The cheers for the winner's decisive victory were cut short when Eight Belles, the runner-up, was euthanized on the track minutes after the race when she collapsed with two broken front ankles.

The unbeaten Big Brown took charge when the 20-horse field turned for home. Under the urging of jockey Kent Desormeaux, Big Brown cruised to a 4 3/4-length victory to become the seventh unbeaten Derby winner with his fourth consecutive win. The last one was Barbaro in 2006.

Eight Belles was attempting to become the fourth filly to win the Derby. However, as the horses galloped out around the first turn, the filly fell to the ground and jockey Gabriel Saez jumped off.

An equine ambulance came onto the track, but the filly could not be saved, said Dr. Larry Bramlage, the Derby's on-call veterinarian.

"When we passed the wire I stood up. She started galloping funny. I tried to pull her up. That's when she went down," a distraught Saez said.