The U.S. Securities and Exchange Commission (SEC) is probing the listings of Slack Technologies Inc and other unicorns on the New York Stock Exchange, the Wall Street Journal reported on Friday, citing people familiar with the matter.

The regulators have asked one of the world's largest hedge funds, Citadel Securities, and at least one other firm for emails and other messages sent on the first day of trading, the Journal reported. 

The SEC had also asked for information from Citadel on how it opened Slack's first day of trading, the report said.

"NYSE places a premium on transparency, fair access and robust price discovery in helping companies access the public markets," NYSE spokesman Farrell Kramer said in a statement.

An SEC spokesperson declined to comment, while Slack (WORK) and Citadel did not immediately respond to Reuters' requests for comment.