U.S. regulators have expanded a probe into the $7.4 trillion mutual fund industry to include certain investment firms’ payments to 401(k) retirement plans, according to newspaper reports on Wednesday.

The U.S. Securities and Exchange Commission has asked two dozen fund firms, including Fidelity Investments and Putnam Investments, to provide details about payments they may make to ensure their funds are included in corporate 401(k) plans that are held by millions of Americans, the New York Times reported.

According to The Wall Street Journal, Lori Richards, director of the SEC’s office of compliance inspections and examinations, said that while there was no evidence that funds had done anything wrong, the agency wanted to look into the issue to head off potential problems.

"We want to make sure that investors, whether they’re investing through a 401(k) plan or directly with mutual fund, that they understand exactly what their money is paying for," the Journal quoted Richards as saying.

For months, regulators have been investigating the fund industry over widespread trading abuses favouring a few larger investors over smaller ones.

Fidelity, Marsh & McLennan Cos. Inc.’s (MMC) Putnam unit, as well as two other large fund firms, Dreyfus Corp. and T. Rowe Price Group Inc. (TROW) , said on Tuesday they were complying with document requests they had received from the SEC, the Times said.

The requests, sent within the last two weeks, are to be met by the end of July and cover details of arrangements at fund companies dating back to January 2002, the Times reported.

The fund firms were not immediately available to comment to Reuters on the reports, nor was a representative of the SEC.

(Reuters)