The Securities and Exchange Commission is investigating the listings of a number of tech unicorns that went public on the New York Stock Exchange in the last few years, including Slack, according to a report from The Wall Street Journal.

The probe is focusing in particular on how trading was handled on the first day these companies went public, the report says.

As part of the probe, the SEC is seeking information from Citadel Securities and GTS, according to a person familiar with the investigation.

It remains unclear which other companies the investigation is looking at, and exactly what kinds of misconduct the SEC might suspect.

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The Securities and Exchange Commission is investigating the listings of a number of highly-valued tech unicorns that went public on the New York Stock Exchange in the last few years, according to a report from The Wall Street Journal.

The probe is focusing in particular on how trading was handled on the first day those companies went public, the report says, though the Journal says that it could not learn what kind of misconduct in particular the SEC may suspect.

The only company specifically named as one of the unicorns the SEC is looking at in its probe is Slack – the workplace messaging company that went public via direct listing earlier this year.

As part of the probe, SEC staff has sent a letter to Citadel Securities asking for information on how it opened Slack’s stock for trading on June 20, when its direct listing hit the markets, the report says. The SEC has asked Citadel Securities and at least one other firm for messages and emails sent before the stocks opened for trading and their policies for following the NYSE’s rules, sources told The Wall Street Journal.

A source familiar with the investigation tells Business Insider that Citadel Securities and another trading firm, GTS, got requests from the SEC for information around a range of IPOs and direct listings that look place at NYSE over the last 5 years.

A spokesperson for the NYSE tells Business Insider that the „NYSE places a premium on transparency, fair access and robust price discovery in helping companies access the public markets.“

A spokesperson at Citadel Securities tells Business Insider, „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.“

The SEC and GTS declined to comment and spokespeople for Slack were not immediately available for comment.

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