An initial autopsy into the death of pre-race Melbourne Cup favourite Admire Rakti has shown the Japanese horse died from a heart attack.

But it could take weeks to determine the reason for that attack, and there is a chance the precise cause of death may never be known.

Admire Rakti collapsed and died after Tuesday’s running of the $6.2 million race, while local stayer Araldo injured a leg in a fence post-race and had to be put down at the University of Melbourne Werribee Vet Hospital a few hours later.

• Cup deaths a tragedy that racing can’t spin 
• Anger over Melbourne Cup deaths

 Dr Chris Whitton, an equine vet at the hospital, said even a full autopsy may not reveal the reason for the death.

“One of the problems we have is that in about half of the horses that die sudden deaths like that, even with a full post-mortem, we don’t get an answer,” Dr Whitton told the ABC.

“In that other half where we get an answer, some of them are due to haemorrhage into the lungs and some of them are due to heart failure, sometimes for reasons again that we don’t really understand very well.”

Dr Whitton said bleeing into the lungs – quite common in racehorses – could have possibly caused the heart attack.

“Haemorrhage into the lungs happens in nearly all horses that race to some degree,” he said.

“It’s usually a small amount and of no consequence. But in some horses it happens to a greater degree and very occasionally and very rarely so much that they actually die.”

“And the reason it happens is because horses are pushing themselves to the limit of biology. They’ve got a very great need for oxygen to their muscles and to do that they have a very high blood pressure in their lungs. The high blood pressure means that there’s a risk of those vessels bursting.”

Racing Victoria’s chief steward Terry Bailey told the ABC there had been no signs pre-race that Admire Rakti was in distress.

“One of the advantages of the internationals is being based at Werribee at the quarantine (centre), these horses are (looked over) by vets everyday and the horse was even examined again on race morning by our vets and trotted up fine and fit as a fiddle,” Mr Bailey said.

“These horses get better looked after than most humans. They have the best veterinary treatment under the sun.

“I’m very comfortable with how our trainers look after their horses.”