Federal regulators sent letters to numerous private equity firms in December in an "informal inquiry" into the $1.2 trillion industry, which historically has not been a major focus of scrutiny by the SEC, the newspaper said.

It was unclear which firms received the SEC letter which "requested information related to 12 broad areas, including fund raising and fund formation," it said.

One focus of the SEC inquiry will be how private equity firms value investments and discrepancies in valuations from firm to firm, it said, citing the letter and people familiar with the matter.

The SEC has increased resources policing private equity as the industry has grown since the 2008-2009 financial crisis. Regulators have several cases involving private equity firms that they may soon bring, the newspaper said.

To date, the private equity industry has steered clear of recent trading scandals and was not at the core of the housing market's collapse.

(Reporting by Karl Plume in Chicago; editing by Mohammad Zargham)

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