Sonoma Raceway officials are investigating "a piece of twine tied in what appeared to be a noose," that was found on the track's property on Saturday, president Steve Page said.
"Our staff, on-site business tenants and local law enforcement have been contacted and asked to share any information they may have," Page said in a statement. "Sonoma Raceway takes this incident very seriously and is dedicated to operating a facility that is welcoming to everyone."
Sonoma Raceway holds 47,000 fans, hosting NASCAR Cup Series events as well as the IndyCar Series. It was closed over the weekend when the noose was found.
"Whoever it was, you're not going to take away my smile," Wallace told the crowd at Talladega. "I'm going to keep on going."
Wallace, the Cup Series' only Black driver, finished 14th on Monday as he drove the No. 43 car for Richard Petty Motorsports. Petty attended Monday's race in his first NASCAR appearance since March.