Federal prosecutors in New Jersey have subpoenaed Gov. Chris Christie’s re-election campaign for documents relating to government meetings that were allegedly canceled with Jersey City’s mayor after he declined to endorse the governor, according to people familiar with the matter.

The subpoenas, sent by U.S. Attorney Paul Fishman, are another sign that the yearlong probe into former allies of Mr. Christie has broadened beyond matters directly related to the George Washington Bridge lane closures.

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