When the Clinton Foundation in 2014 was seeking a firm to manage its planned $250 million endowment, it turned to a familiar source: a New York investment fund with a managing director, Nicole Fox, who is a longtime friend of Chelsea Clinton and served as matron of honor at Ms. Clinton’s wedding.

Ms. Fox, along with the chief executive of Summit Rock Advisors L.P., presented to the foundation during the hiring process, said Clinton Foundation spokesman Craig Minassian.

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