FILE PHOTO: The U.S. Securities and Exchange Commission logo adorns an office door at the SEC headquarters in Washington, June 24, 2011. REUTERS/Jonathan Ernst/File Photo

(Reuters) - The U.S. securities regulator has asked at least two trading firms for information regarding a broad investigation into initial public offerings and direct listings going back five years on the New York Stock Exchange, a person familiar with the matter told Reuters on Friday.

The U.S. Securities and Exchange Commission has asked for documents from Citadel Securities and GTS, the source said.

Earlier in the day, the Wall Street Journal reported that the SEC was probing listings of Slack Technologies Inc and other unicorns on the NYSE, focusing on their first day of trading.

The SEC had asked Citadel Securities for information on how it opened Slack's first day of trading, the Journal reported on.wsj.com/2EDYDlh.

“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 spokesman Zia Ahmed said.

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

An SEC spokesperson and GTS spokesman Rich Myers declined to comment, while Slack did not immediately respond to a Reuters’ request for comment.