The attorney general, Eric T. Schneiderman, has in recent weeks subpoenaed more than a dozen firms seeking documents that would reveal whether they converted certain management fees collected from their investors into fund investments, which are taxed at a far lower rate than ordinary income.

Among the firms to receive subpoenas are Kohlberg Kravis Roberts , TPG Capital, Sun Capital Partners, Apollo Global Management, Silver Lake Partners and Bain Capital, which was founded by Mitt Romney, the Republican nominee for president.

Representatives for the firms declined to comment on the inquiry.

Mr. Schneiderman’s investigation will intensify scrutiny of an industry already bruised by the campaign season, as President Obama and the Democrats have sought to depict Mr. Romney through his long career in private equity as a businessman who dismantled companies and laid off workers while amassing a personal fortune estimated at $250 million.

Some executives at the firms said they feared that Mr. Schneiderman, a first-term Democrat with ties to the Obama administration, was seeking to embarrass the industry because of Mr. Romney’s roots at Bain.

Others suggested that the subpoenas, which were issued by the attorney general’s Taxpayer Protection Bureau, might be part of an effort to recover more revenue for New York under state tax law. The attorney general’s office does not have the power to enforce federal tax laws.

A spokesman for Mr. Schneiderman declined to comment.

The tax strategy — which is viewed as perfectly legal by some tax experts, aggressive by others and potentially illegal by some — came to light last month when hundreds of pages of Bain’s internal financial documents were made available online.

The financial statements show that at least $1 billion in accumulated fees that otherwise would have been taxed as ordinary income for Bain executives had been converted into investments producing capital gains, which are subject to a federal tax of 15 percent, versus a top rate of 35 percent for ordinary income.

That means the Bain partners saved more than $200 million in federal income taxes and more than $20 million in Medicare taxes.

The subpoenas, which executives said were issued in July, predated the leak of the Bain documents by several weeks and do not appear to be connected with them. Mr. Schneiderman, who is also co-chairman of a mortgage fraud task force appointed by Mr. Obama, has made cracking down on large-scale tax evasion a priority of his first term.

As a retired partner, Mr. Romney continues to receive profits from Bain Capital and has had investments in some of the funds that documents show used the tax strategy.

The campaign issued a statement saying that Mr. Romney did not, however, benefit from the practice.

“Investing fee income is a common, accepted and totally legal practice,” said R. Bradford Malt, a lawyer for Mr. Romney who manages his family’s investments and trusts. “However, Governor Romney’s retirement agreement did not give the blind trust or him the right to do this, and I can confirm that neither he nor the trust has ever done this, whether before or after he retired from Bain Capital.”

Managers at a typical private equity firm or hedge fund collect from their investors management fees based on the size of the fund. But most of their compensation comes as a share of the profits earned by the fund. The Internal Revenue Service allows those profits to be considered “carried interest,” taxed at the capital gains rate typically reserved for investments.