WASHINGTON — The Justice Department’s antitrust investigation has found dozens of firms and individuals conspired for at least six to eight years to secretly control the bidding for municipal bond investment and other contracts in more than 250 transactions, according to documents filed with a federal court in Manhattan.

The department has identified 25 entities and 30 individuals as co-conspirators in the scheme, Rebecca Meiklejohn, the department’s lead antitrust attorney in the case, told Judge Victor Marrero, of the U.S. District Court for the Southern District of New York, in a recent five-page letter that included several exhibits.

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