You're right. The autopsy reports, which declared the cause of death (but not the manner of death) were conducted immediately after April 19th. The remains were placed in the trailer for later analysis.
But there was enough information in the autopsy and toxicology reports to show that these folks did not die at their own hands and they died prior to April 19th.
For example, Jimmy Riddle had several gunshot wounds and his body was found in the front of the building, near the kitchen. Yet, he had no trace of tear gas in his system. His body was mutilated by a tank tread.
The only logical conclusion is that he was already dead (by gunshots) before the tanks entered the building and started spraying CS, which means he had already been shot dead before the morning of April 19th.