NASCAR sent a memo to its teams telling them to clamp down on "pockets of complacency" when it comes to following COVID-19 guidelines.
In the note obtained by WCNC Charlotte, NASCAR said it was "seeing more and more vanloads of crew members rolling up to the track without masks on, and people wearing their mask down around their chin."
NASCAR added that if the rules were not followed it "will threaten our ability to continue racing."
The note was supposed to be communicated by teams to everyone in their respective organizations, as COVID-19 cases continue to rise in southern states where NASCAR races and operates.
"More people in our industry are going to contract the virus," NASCAR said, "The key is limiting it."
"It is important for everyone to do their part ALL THE TIME. One cluster outbreak can derail our season."
Seven-time Cup Series champion Jimmie Johnson sat out Sunday's race because of a positive COVID-19 test, but has been cleared to return.