Ending months of anticipation and speculation, Mayor Bill de Blasio will meet on Friday with federal prosecutors and F.B.I. agents who have been investigating the mayor’s campaign fund-raising for nearly a year, according to people with knowledge of the matter.

A grand jury has heard evidence in the case, some of the people have said, but it remains unclear whether the investigation, focused on whether the mayor or others in his administration traded beneficial city action for donations to his 2013 election campaign or to his now-defunct nonprofit political group — or both — will result in charges. Either way, the interview is an indication that the expansive criminal inquiry is most likely in its final stages.

The session is expected to take place at the offices of Mr. de Blasio’s lawyer, Barry H. Berke of Kramer Levin Naftalis & Frankel, familiar ground to the mayor, who has held fund-raisers and meetings there, several of the people said. The meeting had originally been scheduled for two weeks ago, but prosecutors postponed it for reasons that remain unclear.

In recent weeks, investigators appear to have focused on a relatively new area in the inquiry, looking into the mayor’s relationship with a Brooklyn businessman who hosted a fund-raiser for him in October 2013, after the Democratic primary but before the general election, according to two of the people. Like others interviewed for this article, they declined to comment because they were not authorized to discuss the continuing investigation.