NASCAR has indefinitely suspended Penske Racing driver AJ Allmendinger after a second drug test confirmed the presence of "a banned substance" in his system. Race officials have not clarified exactly which substance was found in Allmendinger's urine samples, but a spokesperson for the driver said he has contracted an independent lab to test the products he uses on a regular basis to see if there's a reason behind the positive result. The Allmendinger camp says the results were within nanograms of accepted parameters, though NASCAR has declined to comment on the levels themselves.

Meanwhile, the driver took to Twitter to assure fans that he "will do whatever it takes to get to the bottom of this and get back out there no matter what." Previously, Allmendinger had tested positive for a stimulant back in June and was temporarily suspended on July 7, but denies knowingly taking any banned substances.

At this juncture, Allmendinger will only be reinstated as a driver once he completes a "Road to Recovery" program. NASCAR will issue a letter on that program, and the driver will have to agree to those terms in order to take part.

In the interim, Sam Hornish, Jr. will take Allmendinger's seat at Indianapolis for the next race.