US Attorney Preet Bharara now has Mayor de Blasio in his cross hairs — investigating his campaign fund-raising activities as part of a widening probe into NYPD corruption, sources said yesterday.

The feds are looking at how the mayor solicits campaign cash from members of the real-estate industry — and the fund-raising activities of his former campaign treasurer, Ross Offinger, the sources said.

A source who dealt with Offinger told The Post he “plays fast and loose” with campaign-finance regulations.

The Post revealed on Tuesday that de Blasio took campaign contributions from Jona Rechnitz, a real-estate investor suspected of giving high-ranking cops ­expensive gifts in exchange for favors.

Rechnitz and Jeremy Reichberg, who both served on the mayor’s inaugural committee in 2013, are at the center of the NYPD ­investigation.

Rechnitz donated $50,000 to de Blasio’s nonprofit group, the Campaign for One New York, and Rechnitz and his wife shelled out the maximum $9,900 to the mayor’s 2013 campaign, records show.

In addition, Rechnitz was one of the biggest bundlers for de Blasio’s campaign, raking in more than $40,000 from contributors.

Hizzoner made his first visit to Borough Park after taking office in 2014 to Reichberg’s million-dollar-plus home for a Campaign for One New York fund-raiser.

De Blasio campaign operatives said they will give back the donations from Rechnitz and his wife but keep the more than $40,000 in bundled contributions.

“We are fully confident that the campaign has conducted itself legally and appropriately at all times,” campaign spokesman Dan Levitan said.

Earlier this week, de Blasio said he wouldn’t make any “final judgments” regarding Rechnitz’s donations until the investigation is complete. Asked why de Blasio changed his mind Friday, a City Hall source said, “You’ll have to ask the mayor.”

Hizzoner also does not plan to return the $50,000 that went to the Campaign for One New York.

The mayor announced last month that he was disbanding the Campaign for One New York after good-government groups called for an probe into whether it was ­violating campaign-finance and conflict-of-interest laws.

De Blasio insisted that the group had simply accomplished its mission.

The $102,300 that Rechnitz, who runs the real-estate firm JSR Capital, gave to Senate Democrats at de Blasio’s urging will not be returned, either.

Another politician, Sen. Adriano Espaillat, a congressional hopeful, confirmed Friday that he is returning more than $14,000 in donations he received from Rechnitz.

Offinger was also treasurer of the nonprofit and once served as a campaign-finance director for Sen. Kirsten Gillibrand.

Additional reporting by Kirstan ­Conley and Rich Calder