By Jenna Fryer

DAYTONA BEACH, Fla. (AP) _ Jimmie Johnson and Terry Labonte had their Daytona 500 qualifying runs thrown out Sunday after their cars failed inspection.

NASCAR officials said someone illegally altered the rear window on Johnson's Chevrolet after it passed through pre-qualifying inspection. The infraction was discovered when the car was presented after its qualifying run for a second inspection.

Labonte's car was found to have illegal modifications to the carburettor. The two-time NASCAR champion is driving in the inaugural race for Hall of Fame Racing, the team owned by former NFL quarterbacks Troy Aikman and Roger Staubach.

``These were serious infractions,'' NASCAR spokesman Jim Hunter said. ``I don't think we would disallow the times if this was not serious.''

The penalties are not that serious _ for now.

Both drivers will have to start from the rear of the field during Thursday's qualifying races. The outcome of those races determines the starting lineup for Sunday's season-opening Daytona 500.

But Hunter left open the possibility of further penalties, including confiscation of the cars, suspension of the crew chiefs, monetary fines and maybe even a points deduction.

NASCAR will undoubtedly take a long look at crew chief Chad Knaus, who has been penalized repeatedly for rules infractions on Johnson's team. He was suspended for four races last season when Johnson's car failed a post-race inspection in Las Vegas, but an appeals committee overturned the punishment.

Knaus also has been fined on numerous occasions, and was the first crew chief to lose points when NASCAR began upping its punishments.

Hunter insisted NASCAR would look at this latest infraction individually and not judge Knaus as a repeat offender. Knaus did not immediately return a phone call from The Associated Press for comment.

``Every situation is different, but this is the first event of a new season and I think we want to start the season with a very thorough inspection and I think we did that today,'' Hunter said. ``As far as any prior (infractions), this one is judged by what happened in today's qualifying procedure. We've dealt with those and issued penalties.''

Johnson had originally qualified fifth, Labonte was 28th. Those times meant little in this one event of the season, because only the top two Daytona 500 starting spots were locked up in Sunday's session. The rest of the results were used to determine the starting lineup for the qualifying races.

Both drivers will start those races in the back.