The investigation reportedly focuses on the first day of trading of work-messaging company Slack and a few other unicorns, companies with a valuation above $1 billion, on the New York Stock Exchange, and how the initial public offerings were handled, sources familiar with the matter told WSJ. The NYSE is owned by the Intercontinental Exchange.
The SEC requested electronic messages and emails from right before Slack started trading from electronic-trading firm Citadel Securities, the WSJ said.
Slack went public on June 20 in a direct listing. The SEC declined to comment.
"The direct listing of slack was a tremendous success for the company, its shareholders and our nation's capital markets. From our vantage point, we stand firmly behind the integrity and transparency of the listing and pricing process on this important transaction," Citadel Securities spokesperson Zia Ahmed told CNBC.
The Journal later added that GTS, another trading firm, received a similar request from regulators asking for messages and NYSE compliance policies, citing people familiar with the matter. A spokesperson for GTS had no comment when reached by CNBC.
Slack didn't immediately respond to CNBC's request for comment.
— read the full Wall Street Journal story here.
— with reporting from CNBC's Ryan Ruggiero.