Marching Song, trained by Andy Turnell, and Fenix Two, trained by Jonjo O'Neill, collapsed and died in the ring before the first race on Saturday.
Two other horses, Kid Cassidy and The Merry Giant, appeared to also have been affected, but survived.
The meeting was abandoned after the running of the first race.
The initial suspicion is that the horses died from an electric shock, but authorities have stressed that all possible causes are being examined.
The operations director for Southern Electric Power Distribution, Stuart Hogarth, described the incident as unprecedented and said a full investigation was already under way.
"A section of electricity cable has been removed from the paddock area for further detailed inspection," he said.
Hogarth said initial on-site investigations confirmed there had been no danger to the public.
Stephen Higgins, joint managing director of Newbury racecourse, said the racecourse would wait for further information from the power company.
"We have been reassured that the site is safe and we hope that the final results of this investigation will be established as soon as possible," he said.
Fenix Two and Marching Song were about to be mounted by their jockeys on the grass on the far side of the parade ring when they collapsed.
Reports indicated that several people had reported getting shocks off the horses.
The two horses that died were wearing steel shoes. The two who survived were wearing aluminium racing plates. Both metals are good conductors of electricity.
Higgins said course officials had dug out 40-year-old plans and determined that a power cable had been buried in that area.
He said routine maintenance had been carried out before Saturday's meeting and the inquiry would look at whether that may have in some way contributed to events.
British Horseracing Authority chief executive Nic Coward described it as a tragic incident.
"We should stress that racecourses have a very strong track record of dealing with all of these issues at the track in the interest of welfare - the horse, the jockey, trainers, owners and the customer.
"I think we should look at this as a bizarre moment that people who have been around racing for decades and decades have never seen the like.
"Courses are very vigilant and owners and trainers are very vigilant always. People across the country have seen this happen and their feelings will be for the connections of the horses."
British Horsracing Authority media relations manager Paul Struthers said there were widespread rumours of affected horses having burn marks, but, to his knowledge, no vets, trainers or jockeys had made any such mention in official inquiries to date.