The FBI and the US Justice Department are investigating the discovery of a noose in the garage stall of African-American race car driver Bubba Wallace.
Wallace, the sole full-time black driver in US motor-racing organisation Nascar, successfully pushed to ban the Confederate flag from races.
The flag has been a common sight at Nascar circuits but for many it remains a symbol of slavery and racism.
The news follows Nascar's own pledge to investigate the "heinous" act.
In a statement, the US-based organisation condemned the act at Talladega Superspeedway in Alabama.
"Late this afternoon, Nascar was made aware that a noose was found in the garage stall of the 43 team," the organisation said late on Sunday.
"We are angry and outraged and cannot state strongly enough how seriously we take this heinous act. We have launched an immediate investigation and will do everything we can to identify the person(s) responsible and eliminate them from the sport."
It added: "As we have stated unequivocally, there is no place for racism in Nascar and this act only strengthens our resolve to make the sport open and welcoming to all."
On Monday, the Department of Justice announced that its Civil Rights Division would join the FBI in an investigation to determine whether there was a violation of federal law.
In a statement on Twitter, Wallace, 26, described the incident as a "despicable act of racism" that left him "incredibly saddened".
"This will not break me. I will not give in, nor will I back down," he said.
Earlier this month, Nascar banned the Confederate flag from all races. It came amid global protests against the death of George Floyd while in Minneapolis police custody.
Wallace had called for the flag to be banned from racetracks where it had become a common sight, particularly in the sport's southern US heartland.
"Get them out of here. They have no place for them, " he told CNN. "No one should feel uncomfortable when they come to a Nascar race. It starts with Confederate flags."
And that same week, Wallace and his team, Richard Petty Motorsports, unveiled a new colour scheme for his No 43 Chevrolet, with #blacklivesmatter painted over the rear wheels.
However, before Sunday's scheduled racing at Talladega, a small plane flew over the track trailing a Confederate flag and a banner that read "Defund Nascar".
Nascar executive vice-president Steve O'Donnell condemned the display on Twitter.
You won’t see a photo of a jackass flying a flag over the track here...but you will see this...Hope EVERYONE enjoys the race today. pic.twitter.com/ppwP1ggNjt— Steve O'Donnell (@odsteve) June 21, 2020
Alabama Governor Kay Ivey released a statement on Monday, condemning the "vile act".
"There is no place for this disgusting display of hatred in our state," she wrote on Twitter. "Bubba Wallace is one of us."
Several other Nascar stars joined the chorus of those condemning the act, and voiced support of their fellow driver.
"Cannot believe something like this could happen and the hate that one must have to do this," Driver William Byron wrote on Twitter.
NBA All Star Lebron James also took to Twitter, describing the act as "sickening".
"Know you don't stand alone" James wrote, tagging Wallace on Twitter. "I'm right here with you as well as every other athlete."