Kyle Busch and 65-year-old car owner Richard Childress were involved in a physical confrontation after Saturday's NASCAR Camping World Truck Series race at Kansas Speedway, sources told ESPN.
NASCAR spokesman Kerry Tharp confirmed they are looking into it.
Childress approached Busch and struck him with his fist in the garage area after the race, according to the Charlotte Observer.
The fight was broken up and insults were exchanged before Childress put Busch in a headlock and hit him again, the newspaper said.
Busch finished sixth, a spot behind Richard Childress Racing driver Joey Coulter after the two had a spirited late battle for position. Busch drove up beside Coulter's truck following the race and tapped it.
When asked what happened between the two, Coulter joked that, according to the spotters, Busch was just congratulating him.
Bill Janitz, a spokesman for Busch, told The Associated Press in an email that "there won't be any comment tonight." A phone message left for Richard Childress Racing seeking comment was not immediately returned.
RCR driver Clint Bowyer won the Trucks event to snap Busch's three-race winning streak.
Busch got into a scrap with another RCR driver, Kevin Harvick, at Darlington in early May.
The incident occurred after Regan Smith won his first Sprint Cup race. Busch parked behind Harvick on pit road, and the two cars sat there for almost a minute before Harvick climbed out.
Harvick then approached Busch's car and took a swing at him. As Harvick swung, Busch drove away, pushing Harvick's car into the inside retaining wall on pit road.
Both drivers were fined $25,000 and put on four weeks probation by NASCAR.
The Associated Press contributed to this report.