An investigation has been launched after two horses died in the paddock shortly before the start of their hurdles race at Newbury.
Fenix Two and Marching Song collapsed and fell to the ground, with the reasons for their deaths unknown.
Some horses made it down to the start but there were reports that others had "wobbled" on entering the paddock.
Marching Song's trainer Andy Turnell said: "He went straight down. It looks like they've been electrocuted."
Nicky Henderson, the trainer of a third horse, was quick to withdraw his runner Kid Cassidy, but the novice hurdle race went ahead and was won by Paul Nicholls' Al Ferof.
Henderson said: "About a minute before it happened, Kid Cassidy was walking in the same corner of the paddock. I had my back turned but my daughter said 'your horse has gone down'.
"He got back up again and he went to the start. All the horses at the start had their hearts checked and he was fine, but he was desperately keen so I decided to take him out.
"Electrocution is the most obvious possibility. Nobody is to blame really, but I think you'd only really find out when they've done a post-mortem.
"The worse thing is what happened to the horses and it's all very odd."
Jonjo O'Neill, trainer of Fenix Two, said: "Kid Cassidy was in front and he took a turn.
"We thought he was bucking and kicking and he went down on his knees then he seemed to be OK.
"Mine reared up and we couldn't get him back, it was like he was stuck to the ground. It was the weirdest thing I've ever seen in my life."
Al Ferof's owner Paul Barber was in the paddock and said: "The girl [leading] Turnell's said she felt a tingling as she led them round and then Nicky's horse went down.
"They were getting the shocks off the grass, not off the tarmac. I've never seen anything quite like that ever."
Stipendiary steward Paul Barton said: "We are going to hold an inquiry and interview those connected with the horses.
"We are not going to use the parade ring for the rest of the meeting. The horses will parade in the pre-parade ring and leave from that point.
"The veterinary team are in charge of whatever precautions as far as the other horses are concerned. They have checked all other horses in this race."
