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The bulletin, which did not provide the names of the banks or traders under suspicion, warned of “unsophisticated tradecraft” such as hand signals or special ring tones that traders were using to deliver information about impending orders.

The FBI attributes some of the information to its own interviews with former and current employees at a U.S. bank and a Canadian bank, but also cites information from the inspector general’s office of the Federal Housing Finance Agency.

According to a footnote to the bulletin, the source for some of the information was an employee at the U.S. bank in an interview conducted by a special agent with the FHFA, the regulator of Fannie and Freddie.

The memo does not make clear how active or advanced the FHFA involvement is. Officials at the inspector general’s office were not available to comment. Representatives for FHFA, Fannie Mae and Freddie Mac declined to comment.

Fannie and Freddie, which are government-sponsored enterprises, often do large swap trades to hedge their huge holdings of home mortgages against swings in the bond market. The size of the orders makes the GSEs lucrative targets for front running and market manipulation, the FBI bulletin said.

Front running occurs when someone with advance knowledge of another market participant’s plan to make a sizable transaction puts an order in first, often profiting from a market move that can occur once the big trade has gone through.