“We want to understand how we can promote more transparency” and how electronic trading is “either contributing or not contributing to transparency,” said Goodman, who declined to confirm the existence of any examinations.

The SEC has contacted providers including Tradeweb Markets LLC and TMC Bonds LLC, seeking information about the systems they manage that allow dealers to buy and sell debt to one another and to investors, according to two people with direct knowledge of the matter who asked not to be identified because the conversations were private.

Bloomberg LP, the parent of Bloomberg News, competes with Tradeweb in some businesses, including bond trading with institutional investors. Bloomberg’s trading platforms aren’t geared toward individual clients.

Blocking Rivals

There are reasons some electronic systems enable bond dealers to block rivals from seeing their price quotes, such as if they have found a firm consistently fails to stand by its offers or provides off-market values, said Thomas Vales, chief executive officer of TMC Bonds in New York, who confirmed he’d discussed the issue with the SEC.

Clayton McGratty, a Tradeweb spokesman, declined to comment.

Banks have increasingly turned to electronic systems to sell bonds on behalf of their clients as a way of aggregating a greater number of bids. That’s become more appealing as it’s become more expensive for dealers to use their own money to make markets because of higher regulatory capital requirements.

“There’s a lot of interest in the market given the way it’s grown,” the SEC’s Goodman said.

Rules issued in 2010 by the Basel Committee on Banking Supervision and the Dodd-Frank Act passed by Congress prompted Wall Street dealers to cut their inventories of corporate securities by 76 percent from the 2007 peak through last March, when the Federal Reserve changed the way it reported the data.

At the same time, the size of the U.S. bond market has swelled to $39.9 trillion from $33.6 trillion in 2008 as the Fed held borrowing costs near zero and bought trillions of dollars of Treasuries and mortgage debt, according to data from the Securities Industry and Financial Markets Association.