It is a type of scheme that took down New York’s lieutenant governor last year, and sank the 2013 mayoral campaign of a top Democratic contender: the use of so-called straw donors to funnel illegal contributions to candidates from secret sources.

Now, for the second time, the campaign of Mayor Eric Adams is being scrutinized for the same thing.

On Thursday, the F.B.I. searched the home of Brianna Suggs, Mr. Adams’s chief fund-raiser, as part of an investigation into whether his campaign had received illegal foreign campaign contributions from the Turkish government and Turkish nationals, disguised as coming from U.S. donors who had not actually given their own money, according to a search warrant.

And in July, six men were indicted in Manhattan in connection with a similar scheme, accused of funneling thousands to Mr. Adams’s campaign. Two brothers have pleaded guilty to a misdemeanor conspiracy charge in the case, and the news outlet The City and other organizations have found additional inconsistencies in donations to the mayor’s campaign.

Neither Mr. Adams nor Ms. Suggs have been accused of wrongdoing, and Mr. Adams has denied any knowledge of illegal contributions. But both investigations appear to be focused on whether donors who were eager to get Mr. Adams’s attention sought to mask large donations by funneling them through straw donors — and on who might have coordinated that effort.