The Securities and Exchange Commission is investigating whether Bank of America broke rules designed to safeguard client accounts, potentially putting retail brokerage funds at risk in order to generate more profits, according to people familiar with the enquiry.

For at least three years, the bank used large, complex trades and loans to save tens of millions of dollars a year in funding costs and to free up billions of dollars in cash and securities for trading that Bank of America otherwise would have needed to keep off-limits, these people said.