AJ Allmendinger is "shell-shocked" by his failed drug test and his business manager said Monday that the suspended NASCAR driver is working hard to determine what went wrong.

Allmendinger was suspended by NASCAR just before Saturday night's race at Daytona for failing a random June 29 drug test. NASCAR on Monday received an official request for Allmendinger's "B'' sample to be tested.

"He was a little dumbfounded and shell-shocked Saturday night, and yesterday, it just seemed a little surreal," business manager Tara Ragan told The Associated Press. "It's just so far from AJ's character, and he's trying to come to terms with what has just happened and figure out how this could happen and respect NASCAR's process."

Penske Racing said it is working with NASCAR to "follow its process and procedures," but the team will use Sam Hornish Jr. this weekend at New Hampshire in Allmendinger's No. 22 Dodge. Ragan said Allmendinger is working with Penske Racing to help the organization "get to the bottom of this."

NASCAR and Penske officials have not said what Allmendinger tested positive for, and Allmendinger has made no statement since his suspension.

Allmendinger is the second Sprint Cup Series driver suspended since NASCAR implemented its drug policy in 2009. Jeremy Mayfield chose not to participate in NASCAR's rehabilitation program and instead contested his 2009 suspension in court. He eventually lost his fight after a lengthy battle, and has not raced since.

Allmendinger is the fourth driver spanning NASCAR's three national series to be suspended for a failed drug test; none of the previous three has been re-instated.

He was clearly caught off guard by the test results; Allmendinger's Twitter feed showed he was appearing on behalf of sponsor Shell/Pennzoil less than two hours before the suspension was announced at 6 p.m. Saturday

According to the timeline provided by NASCAR, its medical review officer first alerted Allmendinger of his positive test about six hours before the suspension was announced. Allmendinger then had the opportunity to explain the result, and the medical officer had the responsibility to investigate any offered explanation.

Under NASCAR's procedures, it's assumed Allmendinger was unable to provide a plausible reason for the failed drug test because the second step — alerting NASCAR to the positive result — was done at 2:30 p.m. NASCAR then met with Allmendinger and a senior Penske official, and the suspension was announced roughly 90 minutes before the start of the race.

Hornish was the emergency substitute for Allmendinger on Saturday night. He was pulled off a television set in North Carolina, where he was an analyst on a live broadcast, and flew to Daytona International Speedway. Hornish arrived right before the start of the race. A cut tire contributed to his 33rd-place finish.

Penske Racing president Tim Cindric said Monday the team is again going with Hornish this week because making the decision allows the No. 22 team to move forward with its preparations without any uncertainty.

"Penske Racing does not know when/if B sample test will occur for AJ," Cindric posted on Twitter. "If it does happen, it could take days for results. (We) announced Sam today to ensure we are properly prepared for Loudon as it is more demanding to prepare for than Daytona, not for any other reason."

Hornish is the natural replacement for Allmendinger, who is in his first season with Penske Racing. Allmendinger was hired in late December after the team suddenly split with Kurt Busch.

The 30-year-old former open-wheel driver got the No. 22 seat over Hornish, who joined the NASCAR part of the organization after moving from IndyCar following the 2007 season. Hornish struggled through three full Sprint Cup seasons but spent most of last year out of the car. He made only one Cup start and ran 13 Nationwide races.

This season, Penske has Hornish running the full Nationwide schedule. He has nine top-10 finishes and is fourth in the standings. But he wants to be back in the Cup Series, and was pleased to get the call Saturday to get back to Daytona, where he had finished 10th in Friday night's Nationwide race.

Hornish said it was his first time driving a Shell/Pennzoil car since 2003 in IndyCar. Shell sponsors the No. 22 car, and the company has not commented since the suspension.

Just two weeks ago, Penske officials were adamant that Allmendinger was in line for a contract extension.

Shell has been pleased with Allmendinger's representation, and Penske officials likened his struggles this season to those of Brad Keselowski, who had a rough first season with the team in 2010 but last year produced three wins and a berth in the Chase for the Sprint Cup championship.