Opening a new front in the scrutiny of mutual funds, Massachusetts regulators are investigating whether Putnam Investments defrauded customers by improperly paying rebates to favored retirement plans at the expense of other investors in the big mutual fund company, according to documents and people familiar with the matter.

In one case, Putnam, a unit of Marsh & McLennan (MMC) and the nation’s sixth-largest fund firm, agreed to give the retirement plan of a New York trade union expense-rebate checks of $40,000 a year, according to internal Putnam e-mails, some of which are part of a state regulatory proceeding against the company.

The Massachusetts Securities Division, overseen by Secretary of the Commonwealth William Galvin, is also looking into how the retirement plan of the trade union, Boilermakers Local 5 in New York, used the rebate money, which is intended for administrative expenses for the benefit of the plan’s 1,000 covered employees, according to a person familiar with the matter. Additionally, investigators are examining whether the union trustees of the plan personally benefited from the payments, this person says.

To offer the rebate to the retirement plan, Putnam employees repeatedly said the company would use marketing and distribution fees it collects from all fund shareholders to cover distribution costs, e-mails show. That is significant because the Securities and Exchange Commission has warned the fund industry that rebating such fees to favored customers could be a violation of laws meant to ensure the equal treatment of investors.

In a statement, Ralph Derbyshire, Putnam’s chief lawyer for its retirement plan business, said: “We occasionally reimburse plan sponsors for reasonable plan administration expenses that could otherwise be charged directly to the plan participants. He added that the “reimbursements are paid out of Putnam’s general corporate assets and not out of” marketing fees “payable by other mutual funds to Putnam.”