© Alan Crowhurst/Getty ImagesThe first race at Newbury went ahead following the incident with Harry Skelton riding Al Ferof to victory.
The case is being probed by the British Horseracing Authority, who have announced a full investigation.
The horses, Fenix Two and Marching Song, were being paraded before the first race when they fell to the ground for reasons yet to be explained.
The racecourse later announced that the remainder of the day's racing had been abandoned as a precaution.
"This is a very sad day and our thoughts are with all the connections of the two horses who died," racecourse joint-managing director Stephen Higgins said in a statement.
He later told BBC Radio 5: "We've contacted the local authority.
"We've got the electricity board here having a look but until they've both completed their investigation we're not any clearer.
"It's just one of the most bizarre things that most of us here have ever seen," he added.
Professor Tim Morris, the BHA's director of Equine Science and Welfare, said in a statement: "Whilst there are suspicions that an electrical fault was the cause and this is being looked into by the racecourse and relevant authorities, it is important that we investigate other possible causes."
Samples from the horses involved are to be analysed as well as the water supply.
Former champion jumps jockey and Channel 4 Racing pundit John Francome said: "Three lads involved looking after the first horse that came down said they got a pulsing feeling coming through the horse.
"It seems it was something electrical."
Ex-champion trainer Nicky Henderson withdrew his runner Kid Cassidy. The horse had also buckled in the paddock but recovered and went to the start where the runners were all checked out.
Kid Cassidy was said to be fine but Henderson thought the horse "desperately keen" and withdrew him.
Henderson told BBC Radio 5: "It all happened in exactly the same spot in the parade ring.
"It was all at one end when the horses went on the grass and as you know horses have metal shoes and they puncture into the ground.
"Whether there is a (electric) line through there I've no idea but I'm sure someone is going to tell us," he added.
"But I know it has been mentioned that when they were taking the saddles off the horses they were getting shocks off them."
The first race was run but the remaining six called off.
Source: Reuters
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