NASCAR legend Kyle Busch died from severe pneumonia, which progressed into sepsis ... according to a statement from his family.

The AP reports, according to a family statement, Busch passed away "after severe pneumonia progressed into sepsis, resulting in rapid and overwhelming associated complications."

The outlet reports the family received the medical evaluation on Saturday, citing a news release from Dakota Hunter, VP of Kyle Busch Companies.

As we reported ... Kyle died Thursday after being hospitalized with a severe illness.

Before being taken to the hospital, Kyle was in a North Carolina racing simulator Wednesday when he reportedly became unresponsive. TMZ Sports obtained audio from the 911 call ... where someone told dispatch the patient was "coughing up some blood" at the NC training facility while looking like they were going to pass out.

Kyle's death came 11 days after a NASCAR Cup Series race in which he radioed to his crew and asked for a doctor to give him a "shot" after he finished the race.

On TV, they noted Kyle had been battling some sort of sinus cold that was made worse by the intense G-forces and elevation changes at New York's Watkins Glen course. That race was on May 10.

Despite the health issue, Kyle kept competing ... winning a Truck Series race on May 15 and then competing in the NASCAR All-Star Race on May 17, just four days before his death.

Kyle was 41.

RIP