Hickstead and Eric Lamaze. © Kit Houghton/FEI
Further tests are to be conducted to establish the cause of the rupture, but these tests will take several weeks to complete and may prove inconclusive.
Hickstead was cremated in Italy following the necropsy.
"The sudden loss of any horse, but especially one with the character and history of Hickstead is a shock to everyone, but our thoughts are especially with Eric Lamaze and John Fleischhacker at this difficult time", said FEI Veterinary Director Graeme Cooke.
"Under FEI regulations any equine death at an FEI event must be investigated and a post mortem undertaken. The preliminary findings have revealed a catastrophic rupture of the major blood vessel, the aorta, which very rapidly led to heart failure.
"This is an unusual incident in jumping and while we are investing the very many possible causes of a rupture of this nature, it is possible we may never know the real reason behind it," Cooke said.