Brokers are frustrating efforts by regulators to bring transparency to the $2.7 trillion market for federal agency debt, keeping bond investors in the dark. The second-largest underwriter of debt from issuers such as Fannie Mae, First Horizon National Corp., of Memphis, Tennessee, is among firms that don’t report trades through the Financial Industry Regulatory Authority. Finra, the non-governmental regulator for almost 4,700 brokerages in the U.S., began requiring eight months ago that brokers post transactions through its bond-price reporting system within 15 minutes. About a dozen banks aren’t participating in Finra’s plan, said Mike Nicholas, chief executive officer of the Bond Dealers of America. They may undercut expanding the Trade Compliance and Reporting Engine, or Trace, to more types of debt, including the $5.3 trillion market for mortgage bonds guaranteed by Fannie Mae and Freddie Mac or federal agency Ginnie Mae, which provide more than 90 percent of the funds for U.S. home lending, according to industry newsletter Inside Mortgage Finance.