The Kentucky Derby is just two days away, but the annual event at Churchill Downs has been overshadowed by the death of four horses over a span of five days in Louisville.

Two horses trained by Saffie Joseph Jr. collapsed on the track and died over the past week, while two others died as a result of either racing or training, according to Associated Press.

“While a series of events like this is highly unusual, it is completely unacceptable,” Churchill Downs Racetrack said in a statement on Wednesday.

“We take this very seriously and acknowledge that these troubling incidents are alarming and must be addressed. We feel a tremendous responsibility to our fans, the participants in our sport and the entire industry to be a leader in safety and continue to make significant investments to eliminate risk to our athletes.”

No cause of death was provided for either of Joseph's horses, which were 4-year-old Parents Pride and 5-year-old Chasing Artie. Both horses died after a race over the last five days, per AP.

“When you don't know something, that's when it worries you the most,” Joseph told reporters at the track on Wednesday. “Something is wrong. A lot of thoughts run through your head, but you can drive yourself insane. But I'm very uneasy right now. It's not something I would wish on anybody.”

Lisa Lazarus, the CEO of the Horseracing Integrity and Safety Authority, echoed those sentiments on Thursday.

“When horses die unexpectedly, we all suffer, but we take comfort in the tools and practices we have collectively developed to investigate contributing factors and deploy those learnings to minimize future risk,” Lazarus explained, per AP. “HISA also intends to conduct its own in-depth analysis of the fatalities and will share those findings once the full investigation is complete.”

Although California and New York, which hosts the Belmont Stakes, both have thorough reporting requirements and databases cataloging horse injuries and fatalities, the Kentucky Derby has not yet adopted those measures.

The 149th renewal of the “Greatest Two Minutes in Sports” will run this Saturday, May 6.