The US Securities and Exchange Commission is investigating allegations that Wells Fargo, Morgan Stanley and other Wall Street groups have been systematically cheating customers out of billions of dollars of interest payments.

The probe into “cash sweep” accounts comes as a number of companies have increased the interest they are paying clients. But the rates being offered to savers, particularly by the nation’s largest banks, remains far lower than the returns paid to banks based on short-term interest rates set by the Federal Reserve.

The issue has arisen from idle cash sitting in customer accounts at brokerage firms and large banks, which “sweep” otherwise uninvested funds into interest-bearing alternatives in order to generate income. The SEC is looking into whether the groups steered those clients into sweep accounts that paid little or no interest, and whether the financial advisers at those companies had a fiduciary duty to advise clients they could make higher returns if they moved their cash into other accounts.