The death of Celebre D'Allen has been put down to complications following a lung infection according to the post-mortem.
Philip Hobbs and Johnson White's horse led the Grand National field two from home last Saturday but weakened quickly after that and jockey Micheal Nolan dismounted the 13-year-old after the last.
Shortly after that the horse collapsed and he received treatment overnight at Aintree before being transferred to a nearby stud farm. His condition worsened Monday night and he died early on Tuesday.
A statement on the BHA website said of the post-mortem results: "The report has been shared with the trainer and owner, who have granted permission for the key findings to be published by the British Horseracing Authority (BHA).
"The post-mortem was carried out by Rossdales Veterinary Surgeons of Newmarket.
"The findings of the post-mortem state that the exercise-associated episode experienced by the horse after the race had concluded by the time of death.
"However, a severe bacterial respiratory infection (pleuropneumonia) had occurred post-race which led to the horse’s deterioration on Monday evening, with the subsequent onset of sepsis or endotoxaemia (the release of harmful substances into the bloodstream from bacteria) likely to have been a key factor in the cause of death.
"Blood tests taken from the day of the race indicate that the infection had not been present in the horse on raceday, and was therefore developed after the race.
"Further bloods taken on the Monday indicated a severely compromised immune system. These indications had not been present in the bloods taken on the day of the race. This indicates that this issue emerged subsequent to the race and the exercise-associated episode.
"The heart pathology found no issues which are likely to be significant in the death of the horse."
