America's largest labor federation urged market regulators on Wednesday to force Blackstone Group LP to disclose more information as the huge private equity firm prepares to go public in an offering worth more than $4 billion.

In its second letter to the Securities and Exchange Commission, AFL-CIO said Blackstone should be required to register under the Investment Company Act and follow the same rules that govern mutual funds, which require more disclosure.

The letter was sent two days after Blackstone revealed in a filing with the SEC that its co-founders stand to gain at least $2.33 billion after the firm goes public.

On top of the concern raised by the union, the Blackstone offering has piqued the interest of some congressional lawmakers.

The union said Blackstone accounted for its assets using a substantially different method in an amended prospectus filed on Monday, compared with the company's earlier filings.

The new accounting method does not change the fact that more than 40 percent of Blackstone LP's assets are investment securities and that, as a result, Blackstone LP must register as an investment company, the labor federation wrote. The letter was sent to the SEC's director of corporate finance, John White, and director of investment management, Andrew Donohue.

Blackstone said in a filing that its listed entity was a partnership that exempts it from the governance requirements of investment companies. Those requirements include a fiduciary duty to stock market investors and a majority of independent directors.

However, AFL-CIO said the latest prospectus makes it difficult to determine the nature of the assets held by Blackstone and that its balance sheet remains opaque.

Every week or so Blackstone releases an amended prospectus and each time its asset value is dramatically different, AFL-CIO Secretary-Treasurer Richard Trumka said.

Last week, Blackstone's assets were worth $9.4 billion, this week they're worth $17.2 billion. How will investors know what they're buying in the IPO if Blackstone doesn't even know? he said.

Blackstone's public offering is set for the week of June 25 amid a flurry of leveraged buyouts.

Blackstone earns the bulk of its money through its private equity investing, buying companies with borrowed money and selling them one to five years later. It invests in public and private companies, distressed debt, hedge funds and real estate, among others.

Blackstone's IPO will put part of the company in public hands with unit holders taking a 12.2 percent stake and the Chinese government holding a 9.7 percent share.