Nascar has begun an investigation to find out if its 12-car field for the annual Sprint Cup playoff is tainted.

The questions surround Saturday night’s Federated Auto Parts 400 at Richmond International Raceway, the Cup event that ends the regular season and helps set the field for the 10-race playoff. Nascar wants to know if two teammates deliberately altered the race to help a third make the Chase and in the process knocked two other drivers out of the playoff.

The credibility of Nascar, which is expected to make a decision early this week in advance of Sunday’s playoff opener at Chicagoland Speedway, is the broader question raised by the bizarre ending Saturday.

“It would be bad for the sport if there was clear evidence that a race was somehow manipulated, and Nascar’s very sensitive to this and always has been,” said Ramsey Poston, the president of the communications firm Tuckahoe Strategies and a former Nascar executive.