The chiefs of Robinhood, Reddit, Citadel and Melvin Capital, are headed to Washington for Thursday's highly anticipated GameStop hearing at the U.S.

Robinhood CEO Vlad Tenev, Melvin Capital CEO Gabriel Plotkin, Reddit CEO and co-founder Steve Huffman, Citadel CEO Kenneth Griffin and Keith Gill, also known as Roaring Kitty, will testify to the U.S.

The mania shone a light on the millennial-favored stock trading app Robinhood, which made the unpopular decision to restrict trading of certain securities, including GameStop, during the trading fiasco.

"What we experienced last month was extraordinary, and the trading limits we put in place on GameStop and other stocks were necessary to allow us to continue to meet the clearinghouse deposit requirements that we pay to support customer trading on our platform," Tenev said in testimony he will give that was released on Wednesday.

Tenev cites in his testimony increased capital requirements from the Depository Trust and Clearing Corp., an entity responsible for settling and clearing trades, for the trading restrictions.

Hedge fund Melvin Capital closed out its short position in GameStop after taking huge losses as a target of the army of retail investors.

"I want to make clear at the outset that Melvin Capital played absolutely no role in those trading platforms' decisions," Melvin Capital CEO Gabriel Plotkin said in a testimony released Wednesday.

The hearing's topics are expected to include Citadel's relationship with Robinhood and Melvin Capital, why brokerages shut down trading and how to protect retail investors.

"I want to be clear at the outset: any allegation that Robinhood acted to help hedge funds or other special interests to the detriment of our customers is absolutely false and market-distorting rhetoric," Tenev said in the testimony.

"The existing two-day period to settle trades exposes investors and the industry to unnecessary risk and is ripe for change," Tenev's testimony said.

While Tenev may deflect blame amid his crusade for real-time settlement during the Q&A portion of the hearing, the CEO has already admitted some fault during the trading mania.

However, it is unclear exactly what members of Congress are intending to regulate: brokerages like Robinhood and Charles Schwab, retail investors on social media, market makers like Citadel or hedge funds.

James Comer (R-Ky.) told CNBC he has been proud to see a company like Robinhood emerge, leveling the playing field for retail investors.

Reddit's CEO will likely face questions about the possibility of market manipulation and hate speech on its page WallStreetBets, which was inundated with posts about GameStop during the short squeeze.

Gill — who goes by DeepF------Value on Reddit and Roaring Kitty on YouTube — defended his social media posts that helped spark a mania in GameStop shares last month in testimony released on Wednesday, saying he was an individual investor acting only on publicly-available information.

"My investment in GameStop and my posts on social media were entirely my own," Gill's testimony said.