The noose found in the garage stall of NASCAR's only black driver Bubba Wallace was not evidence of a racist hate crime but a rope that had been there for months, US investigators said Tuesday.

"The FBI has completed its investigation at Talladega Superspeedway and determined that Bubba Wallace was not the target of a hate crime," NASCAR said in a statement after outrage erupted over the incident, at a time when protests against racial injustice are sweeping the United States.

The probe found the garage door pull-rope fashioned like a noose was in the stall long before Sunday's race at Talladega in Alabama.

"The FBI learned that garage number 4, where the noose was found, was assigned to Bubba Wallace last week," Northern District of Alabama Attorney Jay Town and FBI Special Agent in Charge Johnnie Sharp Jr said in a statement announcing there would be no charges.

"The investigation also revealed evidence, including authentic video confirmed by NASCAR, that the noose found in garage number 4 was in that garage as early as October 2019.

"Although the noose is now known to have been in garage number 4 in 2019, nobody could have known Mr Wallace would be assigned to garage number 4 last week."

The FBI investigation, which included 15 agents sifting through evidence and conducting interviews, was launched on Monday, a day after the noose was found.

Wood Brothers Racing, who had a car in that garage for a race last autumn, cooperated in the investigation, saying in a statement on Twitter that one of their employees had seen the rope there.

"We immediately alerted NASCAR and have assisted in the investigation in every way possible," the team said.

Wallace told CNN on Tuesday night that whether it was targeting him or not, the rope was definitely twisted into a hanging noose in the photo he saw -- an image that recalls lynchings of black Americans.

"The image I've seen of what was hanging in my garage was not a garage (door) pull," he said. "It wasn't directed at me, but somebody tied a noose."

Security had been stepped up for Wallace, the only black driver who races full time in NASCAR's top Cup series and who recently successfully campaigned for the banning of the Confederate flag at the popular stock car series events.

NASCAR drivers push the #43 Victory Junction Chevrolet, driven by Bubba Wallace, to the front of the grid as a sign of solidarity with the African American driver prior to the NASCAR Cup Series race Talladega, Alabama
NASCAR drivers push the #43 Victory Junction Chevrolet, driven by Bubba Wallace, to the front of the grid as a sign of solidarity with the African American driver prior to the NASCAR Cup Series race Talladega, Alabama GETTY IMAGES NORTH AMERICA / Brian Lawdermilk

The Confederate flag, seen as a symbol of slavery and racism by many, has been a staple at NASCAR tracks, particularly in the sport's southern US heartland.

Before the scheduled Sunday start of the race at Talladega, the flag was flaunted by some outside the speedway.

However, the weather-delayed race on Monday turned into a mass show of support as drivers closed ranks around Wallace.

He was visibly moved before the start as his fellow drivers rallied behind him at the starting line.

Multiple drivers and crew then joined forces to push Wallace's No.43 car to the front of the grid.

Wallace exited the car and appeared to be in tears as driver after driver hugged him.

"I feel like there's a ton of support," Wallace told CNN. "We've seen everybody come together on Monday, that was one of the coolest things that I've ever been able to be a part of. I'm not saying I wanted that, but drivers wanted to do it."

But he was angered by some social media comments suggesting he had somehow fabricated the incident, stressing he had already left the garage when someone else spotted the noose and alerted NASCAR officials.

"I'm pissed," Wallace said. "I'm mad because people are trying to test my character and the person that I am, my integrity.

"They're not stealing that away from me, but they're just trying to test that."