Simon Cowell's production companies Syco Entertainment and Fremantle, and NBC, which broadcasts America's Got Talent, are being investigated.
It follows the controversial departure of Gabrielle Union, 47, as a judge on America's Got Talent after she raised concerns about the discriminatory workplace culture of the show.
The shows failure to renew her contract is being investigated by Sag-Afrta, which represents actors and performers.
Variety reported that Union had urged producers to report an offensive joke made by Jay Leno while filming the show.
The joke was at the expense of Koreans, and was thought to have perpetuated stereotypes about Asian cultures eating dog meat.
Insiders told Variety a number of people were offended by the joke, and it was never raised to HR but was eventually cut from the show when it aired.
They said it was just one example of the "toxic culture" at the show.
Union also reportedly received "excessive notes" on her appearance, and was even told her hair styles were "too black" more than a dozen times.
Julianne Hough, 31, whose contract was also not renewed, is also alleged to have received excessive critiques of her personal appearance including hair, make up and wardrobe.
Insiders claimed this had a detrimental impact on her morale - though Hough has denied this to Variety.
The sources also reported other incidents which raised concerns for Union and other members of staff, such as a white male auditionee's portrayal of black people during an impersonation which saw him black up his hands.
Actress Elizabeth Banks tweeted her support of Unino and the investigation.
She wrote: "Thank you @itsgabrielleu for speaking up about a toxic culture at @NBC 's #AmericasGotTalent . It’s time for NBC to take these allegations seriously. Join me & @TIMESUPNOW in calling for accountability & concrete action".
Union tweeted: "So many tears, so much gratitude. THANK YOU! Just when you feel lost, adrift, alone... you got me up off the ground. Humbled and thankful, forever".
NBC, Fremantle and Syco released a joint statement on Sunday.
It said: "We remain committed to ensuring a respectful workplace for all employees and take very seriously any questions about workplace culture.
"We are working with Ms Union through her representatives to hear more about her concerns, following which we will take whatever next steps may be appropriate."
America's Got Talent released a statement saying: "We remain committed to ensuring a respectful workplace for all employees and take very seriously any questions about workplace culture.
"We are working with Ms. Union through her representatives to hear more about her concerns, following which we will take whatever next steps may be appropriate."
MirrorOnline has contacted reps for Jay Leno, Syco, Sag-Afrta and Fremantle for comment.
