Days before Kyle Busch’s death, the star NASCAR driver raced with his son at a go-kart track in North Carolina, engaged with fans and maintained his typically competitive demeanor.The driver’s apparent vigor earlier in the week, according to those who interacted with him, contributed to the widespread shock and devastation felt throughout the motorsports community when news broke on May 21 that Busch had died at age 41. The update came less than eight hours after Busch’s family and team announced that he had been hospitalized due to a severe illness, and less than a week after his last NASCAR race win.On Saturday, the Busch family announced that the medical evaluation they were provided with concluded that “severe pneumonia progressed into sepsis, resulting in rapid and overwhelming associated complications.”A death certificate obtained by The Athletic on Wednesday revealed that Busch had bacterial pneumonia for “days to weeks,” which led to multiple other serious issues, including sepsis, before he died.Those who were around Busch earlier last week shared that he was smiling, interacting with others and spending time with his 11-year-old son, Brexton, while racing May 19 at the Andretti Indoor Karting facility in Durham, N.C. Racing legend Mario Andretti said his grandson, Marco, invited Busch to the facility, which was holding its grand opening. He showed The Athletic a video of himself shaking hands with Busch at the event and called the racer’s subsequent death “a shock of my life” and “unbelievable.”“His legacy will be there forever,” Andretti said. “The talent, what he’s done with NASCAR, he is one of the golden boys of NASCAR, for sure. What he has contributed is immense. Everybody knew Rowdy. He’ll bring a smile when we talk about him.”Naz Idoura, an amateur racer and NASCAR fan who spent the afternoon at the facility, said he didn’t realize Busch — a two-time Cup Series champion and the winningest driver in NASCAR national series history, with 234 victories across the Cup, O’Reilly and Truck series — would be there for the event, but he noticed a driver without a helmet racing among a group of kids.“I was like, ‘Oh, it’s Kyle Busch. Oh no, don’t let those kids beat you now!’” Idoura said.“Those kids were impressive,” he continued. “They were literally wheeling those carts around and drifting around the corner, showing off their car control. And Kyle Busch was … guiding them like a father figure. Almost like a mother hen guiding the babies, just kind of teaching them things, pushing them to be better.”Idoura said Busch trailed behind the kids on track and pushed them to go faster, but “you could see a little intensity with him.“He was obviously having some fun, but he would be focusing on the monitor after the race was over,” Idoura said. “I would see him at the monitor, checking lap times out, looking at the telemetry, seeing how (Brexton) was doing.”Earlier in his career, Busch was widely seen as a polarizing figure and NASCAR villain for his brash manner on and off the track. In more recent years, however, he’s spotlighted his role as a husband and father, as well as a racing coach for Brexton, who competes at the grassroots level. Their social media pages often feature videos of the two together around the track.