Jockey William Buick has been taken to hospital in Chicago after a fall from the fatally-injured Permian in the Secretariat Stakes at Arlington Park.
Assistant trainer Charlie Johnston, son of trainer Mark Johnston, said Buick was conscious but complaining of back pain.
Permian had to be put down with a broken leg.
“We’ve lost the horse. He suffered a catastrophic fracture of his left fore,” Charlie Johnston told Press Association Sport.
“I was with him on the track for 30 seconds. He was in the process of running a very disappointing race and I believe it was when he was winding down that it happened.
“I am with William and he is conscious, but complaining of back pain.”
Permian was one of the big success stories of this year’s Classic generation, having won the Dante Stakes at York and the King Edward VII Stakes at Royal Ascot.