CHARLOTTE, N.C. (AP) — NASCAR filed court documents Monday night that show an independent laboratory found an illegal substance in the urine sample that led to Jeremy Mayfield's suspension for a failed drug test.

The documents, part of NASCAR's response to Mayfield's lawsuit to have his indefinite suspension lifted, show that Medtox Laboratories in Minnesota tested both his "A'' and backup "B'' samples last week and "confirmed the presence" of a substance that is blacked out in the filing.