Allmendinger previously had said he tested positive for a stimulant he had not identified, insisting he did not knowingly ingest a banned substance.
Penske Racing president Tim Cindric said the team has no reason to doubt Allmendinger's assertion that he didn't know why he failed the test. It isn't clear whether that will be enough to save his job. In an interview with the Associated Press, Cindric said team officials plan to meet next week to discuss Allmendinger's future.
"Certainly, we have to try to understand how this all plays out and how it affects not only his future, but how it affects a lot of the various programs that we have going," Cindric said. "And it has a knockdown effect in a lot of different ways that we're going to take some time to analyze, and not really have a conclusion to that here in the next few days."
NASCAR suspended Allmendinger indefinitely after a test of his backup urine sample conducted Tuesday confirmed an initial positive test in late June. NASCAR officials did not identify the substance Allmendinger tested positive for.
Allmendinger has hired an independent laboratory to help him determine how he might have tested positive. Cindric said team officials "don't have any reason not to believe" his explanation that he doesn't know what triggered the positive test.
"I think he shares as he understands," Cindric said. "I think there's a process that he has to go through to understand. But on our end, no, I wouldn't say that we're frustrated with his process. Because it's his career that's at stake, and he has to determine what the best process is for him."
Allmendinger's commitment to participate in NASCAR's anti-drug program means he will be evaluated by a substance-abuse professional who determines a path to reinstatement that may include counseling and rehabilitation.
"While we await further information from testing to determine the cause, we have notified NASCAR that A.J. will participate in the Road to Recovery Program starting immediately," according to a statement issued by Allmendinger's business manager, Tara Ragan. "As we have stated earlier, we respect NASCAR's drug testing policies. They are first and foremost in place to protect drivers."