NASCAR star Kyle Busch died from severe pneumonia that progressed into sepsis, according to a statement released Saturday by his family.

"The medical evaluation provided to the Busch Family concluded that severe pneumonia progressed into sepsis, resulting in rapid and overwhelming associated complications," according to the statement from the Busch family. 

Busch, who won more races in NASCAR's top three series than anyone in history, died suddenly on Thursday after passing out a day earlier in a driving simulator at General Motors facility in Concord, North Carolina.

Sepsis is a life-threatening medical condition that occurs when the body has an extreme response to an infection, causing damage to tissues and organs, according to the Centers for Disease Control and Prevention.

This was the Busch family's first statement since Thursday, when they wrote on social media that he "experienced a severe illness resulting in hospitalization" and would miss this weekend's Coca-Cola 600, one of the biggest races of the season.

The 41-year-old experienced shortness of breath, felt he was overheating and was coughing up blood the day before his death, according to a 911 call obtained Friday by CBS News.

During the emergency call, an unidentified caller calmly told the dispatch: "I've got an individual that's shortness of breath, very hot, thinks he's going to pass out, and he's producing a little bit of blood, coughing up some blood."

Busch was lying on the bathroom floor inside the complex and the caller told dispatch, "he is awake," according to audio provided by the Cabarrus County Sheriff's Office.

Two weeks ago, while competing in a Cup race at Watkins Glen, Busch was heard on radio telling his team to have a doctor waiting for him when the race ended. The TV broadcast said Busch had been dealing with a sinus cold.

Despite the issues, Busch won the Craftsman Truck Series race on May 15 at Dover Motor Speedway, less than a week before his death. 

Busch won Cup Series titles in 2015 and 2019 while driving for Joe Gibbs Racing. Busch is the all-time leader in wins in the O'Reilly Auto Parts Series (102) and Craftsman Truck Series (69), considered the second and third tier in the sport respectively. Combined with 63 wins in the Cup Series, he holds the most wins ever by a driver in NASCAR's three top divisions (234).