(CNN)Two racehorses died Saturday at Pimlico Race Course, site of the Preakness Stakes to be held later in the day.
Homeboykris collapsed after the first race of the day while walking back to his barn. The horse, a 9-year-old gelding, won the race at the Maryland track in Baltimore.
Trainer Francis Campitelli was in the stands when his horse went down.
He told The Baltimore Sun, "The boy that takes care of him said they had gone probably 100 yards, and he got wobbly and fell over and he pretty much was dead when he hit the ground."
Campitelli said they thought the horse was in "really good health" and why he died was still a bit of a mystery.
"They're thinking at this point it was some sort of heart attack ... ruptured aorta or something like that," he told the Sun. We won't know until they do a necropsy on him, just to find out exactly what happened."
Pramedya, a 4-year-old filly, fractured a cannon bone in her leg while running on grass in the fourth race and was euthanized on the track. Jockey Daniel Centeno broke his collarbone in the fall.
The horses will have necropsies performed at New Bolton Center Hospital in southeastern Pennsylvania, Pimlico spokesman David Joseph said.
Mike Hopkins, executive director of Maryland Racing Commission, said those tests usually take seven to 10 days.
"It really is unfortunate," he told CNN. "We at the Maryland Racing Commission take safety and integrity very seriously and we conducted thorough examinations and inspections before every race, as well as after every race. We have several veterinarians on-staff and on-site, and we inspect them in the barns, in the paddock, and at the starting gate."
Pramedya is owned by Roy and Gretchen Jackson, who owned Barbaro, the 2006 Kentucky Derby who pulled up lame during the Preakness. He underwent eight months of veternary care but was euthanized in in January 2007. Pramedya had won two of her first four career starts, including one race this year.
Homeboykris had run 62 races before Saturday, winning 13 and finishing in the top three 28 times. He finished 16th in the 2010 Kentucky Derby.
A closer look at racing deaths
People for the Ethical Treatment of Animals called on the horses' owners "to release veterinary records & complete list of medications that horses were administered before #Preakness races."
A 2012 New York Times look into horse racing found that 24 horses die each week in the United States on average. The Times wrote that after Eight Belles was euthanized on the track after the 2008 Kentucky Derby, Congress got the horse racing industry to increase safety for horses and riders. One of the measures was a policy banning many anabolic steroids.
A total of 4,649 thoroughbreds -- a rate of 1.87 for every 1,000 starts -- died in racetrack-related incidents from 2009 to 2015, according to the Equine Injury Database compiled by The Jockey Club. In 2015, the fatality rate was the lowest (1.62) of the seven years for which data was available
"These improving fatality rates are clear evidence that we can move the needle and that the efforts of so many are truly bearing fruit," Dr. Mary Scollay, the equine medical director in Kentucky, said in March.
The Jockey Club, the registry for thoroughbred horses in the United States, Canada and Puerto Rico, said the data include horses that had injuries that caused death within 72 hours of a race. The data doesn't include quarter horses or standardbred horses.
Last week, the National Thoroughbred Racing Association's Safety & Intergrity Alliance announced that it had reaccredited Pimlico Race Course.
"We are proud to once again earn the highest of marks in safety and integrity in the alliance's accreditation," Maryland Jockey Club President Sal Sinatra said.