An official familiar with the probe says New York's attorney general is investigating some of the nation's largest private equity firms, including Bain Capital, founded by Republican presidential nominee Mitt Romney.

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. 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.