However, authorities are little closer to determining what triggered the deaths, with some toxin or poison still considered the most likely cause.
Investigators will now have to wait for toxicology tests, which could be a week or more.
Post-mortem investigations on 15 of the horses were being conducted in Gainesville at the University of Florida College of Veterinary Medicine and the remaining six at Florida's Animal Diagnostic Lab in Kissimmee.
It is understood most of the examinations have been completed.
The Florida Department of Agriculture has confirmed to media that the horses examined to date had suffered bleeding in the lungs. The animals also suffered pulmonary oedema.
Pulmonary oedema is fluid buildup in the lungs that triggers respiratory failure and can be caused when the heart is unable to remove fluids from the lungs.
Investigators are reportedly following up on reports that the horses were injected with a vitamin supplement. The supplement is reported to aid recovery, as opposed to performance.
A report in the Argentine newspaper La Nacion quoted one member of the team as saying that five horses who did not receive the supplement were OK.
Authorities are confident that any medicines or supplements given to the horses would show up in toxicology screening.
The diagnostic lab in Kissimmee is also testing samples of the bedding, hay, feed and water collected from Wellington, where the horses were housed.
As well as tests on body fluids from the dead horses, testing will also be done on samples from polo ponies from the team that were unaffected.
It is understood that blood samples will be tested by the College of Veterinary Medicine's racing lab.
The polo ponies, collectively worth more than $US2 million, began falling ill shortly after arriving at the International Polo of Palm Beach grounds about 2pm on Sunday.
Within hours, 14 of the ponies belonging to the Venezuelan-based Lechuza Caracas team were dead and another seven died died overnight.
Several formal investigations are under way, including state authorities in Florida and the US Polo Association.
The polo team has said it will assist formal investigations in any way possible.
The Lechuza Caracas team is owned by Venezuelan multimillionaire Victor Vargas.