Rider Zac Purton said Melbourne Cup favourite Admire Rakti "shuddered" as he pulled up in a shock last place at Flemington.
Fifteen minutes later one of the world's finest staying thoroughbreds was dead.
It was one of the biggest shocks in 152 years of Melbourne Cup racing and ranks alongside the nobbling and scratching of red-hot Cup favourite Big Philou in 1969.
Hours later, another runner Araldo was put down after the stallion broke a bone in his leg when he was startled by a flag on his way back to scale, kicked out and struck a fence.
Admire Rakti broke into a heavy sweat as he approached the hosing bay at the saddling area of Flemington yesterday afternoon.
He was immediately taken to his tie-up stall where he collapsed. He could not regain his feet and died.
The Victoria Racing Club veterinarians have impounded the remains and an autopsy will be done as soon as possible to determine the cause of death.
Purton said for most of the race there was no sign of trouble.
"I thought before the race he'd win and in running I was certain he'd win. He felt terrific."
Purton surprised many when he took Admire Rakti forward to share the pace in stark contrast to dropping back before storming home to win the Caulfield Cup.
"At the 800m I started to feel that something wasn't quite right with the horse," Purton said. "Then he started to lose ground and I knew we were in trouble.
"The horse always has to come first so I eased him down. When we pulled up he shuddered."
One possible scenario is a massive lung bleed.
Purton said Admire Rakti's owner, Riichi Kondo, was "distraught".
Coalition for the Protection of Racehorses spokesman Ward Young said Admire Rakti's death was another example of horses being overworked on the racetrack.
The Cup was won brilliantly by second favourite Protectionist, ridden by Ryan Moore.
The 3200m time of 3.17.7 is the fastest Melbourne Cup since Kingston Rule won in 1990.
Kingston Rule's trainer, Bart Cummings, made it to Flemington when everyone bet against him doing so. The 86-year-old is frail, but was determined to see 9-year-old Precedence run in his fourth Cup.
Precedence's part owner, Sir Patrick Hogan, met Cummings at the weekend. "He said he's been to every Melbourne Cup he's had a runner in and he didn't want to break that tradition."
Precedence did his part by coming from last to finish sixth.
New Zealand's only claim to a runner, Who Shot Thebarman, ran the race of his life to finish third and pick up A$450,000.
Kiwi trainer Chris Waller was delighted. "He's going to be a better horse next preparation," Waller said.