NASCAR driver Kyle Busch died from severe pneumonia that progressed into sepsis, according to a statement provided by the Busch family Saturday.
Dakota Hunter, vice president of Kyle Busch Companies, said in a news release that the family received the medical evaluation Saturday.
"The medical evaluation provided to the Busch Family concluded that severe pneumonia progressed into sepsis, resulting in rapid and overwhelming associated complications," the Busch family statement read. "The Family asks for continued understanding and privacy during this difficult time."
Busch, a two-time NASCAR champion, died at 41 on Thursday, a day after passing out in a Chevrolet simulator. His family said earlier that he had been hospitalized with a "severe illness" days before he was to compete in the Coca-Cola 600 at Charlotte Motor Speedway.
Busch was testing in the racing simulator in Concord, North Carolina, on Wednesday when he became unresponsive and was transported to a hospital in Charlotte, several people familiar with the situation told the AP.
Busch 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 ABC.
During the emergency call placed late that afternoon, an unidentified caller calmly told the dispatch: "I've got an individual that's [got] shortness of breath, very hot, thinks he's going to pass out and is producing a little bit of blood, coughing up some blood."
The caller said Busch was lying on the bathroom floor inside the complex and told dispatch "he is awake," according to audio provided by the Cabarrus County Sheriff's Office. The man then gave directions on where emergency responders should go and asked that they turn off any sirens upon arrival.
Sepsis is considered a life-threatening medical emergency that occurs when the body has an extreme, overactive response to an infection, causing the immune system to damage its own tissues and organs, according to the Centers for Disease Control and Prevention.
Typically the immune system releases chemicals to fight off pathogens such as bacteria, viruses or fungi, but with sepsis the response goes into overdrive. The results can cause widespread inflammation, form microscopic blood clots and make blood vessels leak.
Busch was thought to have had a sinus cold while racing at Watkins Glen on May 10 and radioed his team saying that he needed a "shot" from a doctor after the race.
Busch won 234 races across NASCAR's top three series, more than any other driver in history.
All 39 drivers in the field for Sunday's race will have a black No. 8 decal on their car to honor Busch.
Information from The Associated Press was used in this report.