AJ Allmendinger has opted to accept NASCAR's "Road to Recovery" program beginning immediately, his business manager Tara Ragan said Wednesday afternoon.

According to Ragan, Allmendinger does not know what he ingested to cause his positive test result for a banned stimulant and is using an independent lab to test substances in his home and motorcoach to see what was responsible for the failed test.

Allmendinger was indefinitely suspended by NASCAR after his 'B' sample also tested positive on Tuesday. NASCAR has not named the substance found in the test.

"As we have stated earlier, we respect NASCAR's drug testing policies," Ragan said. "They are first and foremost in place to protect drivers and AJ being among those. We fully support the program, and as more details become available, we will share them."

In a statement, NASCAR said it was "very pleased" Allmendinger has chosen to participate in the program.

"It's designed, as proven, to provide a road map leading to a return to competition, and we wish him the best of luck," the statement said. "As we have with other competitors, we look forward to the day when the program administrator recommends him for reinstatement."

Now that Allmendinger has accepted NASCAR's terms, he will have to undergo an evaluation by a substance abuse counselor. That meeting will generate a further checklist of goals for Allmendinger. After he completes the program to the satisfaction of the NASCAR program administrator – which could take months – he could then be recommended for reinstatement.

Allmendinger issued his own early-morning statement via Twitter – his first comments about the situation – thanking fans for their support.