Jeremy Mayfield © Gallo Images

A person familiar with the test results told The Associated Press on Thursday that Mayfield's positive test was not for a performance-enhancing drug. The person spoke on condition of anonymity because NASCAR won't reveal what banned substance was found in the random test, which ultimately resulted in Mayfield's indefinite suspension.

NASCAR officials previously announced the drug violation was not alcohol-related, and the administrator of its drug testing program has dismissed Mayfield's explanation that the positive result came from a mix of a prescription with an over-the-counter medicine.

Under the series' toughened policy, that leaves the possibility that Mayfield tested positive for abuse of a prescription drug, narcotics or controlled substances, such as cocaine, marijuana or methamphetamine.

Because Mayfield challenged the initial positive finding, as allowed under NASCAR's drug policy, the series did not take disciplinary action until his B sample also tested positive. That's why Mayfield wasn't barred from participating in two practice sessions and qualifying on May 8 at Darlington.

"There are limitations as to how quickly the process can be brought to conclusion," said Dr. David Black, the administrator for NASCAR's drug-testing program. "The practical reality is there is going to be a delay. In an ideal world, if the world were perfect and there was a possibility of an instant answer, we'd be able to take immediate action."

After Mayfield tested positive and Aegis investigated his explanation and rejected it, Black's office told NASCAR officials on May 7 about the positive test. On May 8, Mayfield showed up at Darlington, ready to get on the track, and asked for his B sample to be tested. NASCAR put a rush on the lab order to learn the results before the Southern 500 on May 9.

While they waited, Mayfield took part in two practice sessions with other cars on the track alongside him.

Black would not speculate if allowing Mayfield on the track put Mayfield or the other drivers in danger.

Mayfield ran 16 laps in the first session, 23 laps in the second and hit a top speed of 173.577 mph (279.33 kph). Mayfield later ran two qualifying laps alone on the track but failed to qualify for Saturday's race.

"Certainly we were in contact with Jeremy that day, and there was no physical reason to believe he couldn't perform," NASCAR spokesman Ramsey Poston said.

NASCAR finds itself in a unique position in its first season under the toughened drug policy. While other major sports leagues must focus on the effects of performance-enhancing drugs on their traditions and records, the abuse of recreational drugs and the altered states they create can present an imminent danger in NASCAR, where 43 stock cars are on the track at once, racing at high speeds in 3,400-pound (1,500-kilogram) cars.

"It's unique in the much greater potential of life and death," said Dr. Gary Wadler, who leads the committee that determines the banned substances list for the World Anti-Doping Agency.

Wadler believes NASCAR would be justified to make its rules even stricter than those in other sports.

"Therefore you have to go the extra mile to fully protect the innocent," he said.

Black said NASCAR's policy leaves a window for an individual to protest the initial failure of a sample and takes into account the possibility of a false positive. Because Mayfield offered an explanation for his positive result, it had to be investigated and dismissed before his B sample was tested.

That created the gap that allowed Mayfield to participate in Friday's on-track activity at Darlington.

Mayfield, who turns 40 this month, can apply for reinstatement only after completing a program designed by Black. His suspension covers both his role as driver and owner of Mayfield Motorsports.

His Toyota will run this weekend at Lowe's Motor Speedway with Mayfield's wife, Shana, listed as the car owner and J.J. Yeley as the driver.

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