A day after learning Jeremy Mayfield failed a drug test for something other than a performance-enhancer, NASCAR allowed him to drive a race car at Darlington Raceway at speeds up to 173 mph.

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 sport's 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 backup "B" sample also tested positive. That's why Mayfield wasn't barred from participating in two practice sessions and qualifying session 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."

Mayfield was first told on May 5 that he had failed a random drug test and was asked to explain why he might have tested positive, according to an outline of NASCAR's procedures provided by Black, CEO of Aegis Sciences Corp. in Nashville, Tenn., which runs the testing program.

After Aegis investigated Mayfield's 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 backup "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.

"We didn't collect a sample that day on the individual, so I can't predict without the test result, to know if the person had consumed the drug of concern," Black said.

Mayfield ran 16 laps in the first session, 23 laps in the second with a fast lap of 173.577 mph. Mayfield later ran two qualifying laps alone on the track but failed to qualify for Saturday night'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. "Dr. Black's team had a rush order to get us the results. They literally worked through the night so we would know the "B" sample before Saturday night's race."

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 cars are on the track at once, racing at high speeds in 3,400-pound 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.

Using banned substances can be dangerous to competitors in other sports — what if in football, the entire offensive line is on steroids and none of the defensive linemen are?

But in auto racing, Wadler said, "it's a different order of magnitude." So he 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 at the end of 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. The No. 41 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.

This season, NASCAR, which previously only tested when there were reasonable suspicions, ordered preseason testing for all drivers and crew members and added random testing throughout the season. NASCAR provided teams last December with a detailed list of banned substances it would test crew members for this season.

No such banned list exists specifically for drivers because NASCAR reserved the right to test for anything it wants above and beyond the baseline crew member list. To Wadler, that undermines the legitimacy of the program.

NASCAR won't disclose what Mayfield tested positive for.

"The reason we don't reveal the substance is because our policy says the misuse or abuse of any substance is a violation," Poston said. "The substance is irrelevant. What's important is that a drug, under a positive test, a drug has been misused or abused."

NASCAR does have the right to reveal the drug because compliance with its testing program does not fall under the guidelines of the federal HIPAA laws, Black said.

"We could make it public," he said. "But the issue up to this point has been to respect the individual's privacy."