The official spoke on the condition of anonymity because of the sensitivity of the probe. It is examining whether the firms used a tax strategy to avoid paying hundreds of millions of dollars in taxes. The practice involved converting some fees collected for managing accounts into fund investments, resulting in a lower tax rate.
The New York Times first reported the investigation of more than a dozen firms.
"Among the firms to receive subpoenas are Kohlberg Kravis Roberts & Company, 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," the Times wrote.
Representatives for the firms declined to comment on the inquiry. Tax experts tell the Times they have differing views about the strategy's legality.
Though Romney collects benefits as a Bain retired partner, the inquiry isn't focused on the time he ran Bain.