Hard-charging US Attorney Preet Bharara sent a message to Mayor de Blasio Friday — trotting out the same prosecutors who took down Sheldon Silver and Dean Skelos to interrogate Hizzoner about his campaign fund-raising activities.

“Preet’s office is a force to be reckoned with, but these prosecutors he rolled out for this case are a big deal and very competent, indicating that they are taking this matter very seriously,” a federal law-enforcement source told The Post.

Andrew Goldstein, chief of the public corruption unit, and his deputy Tatiana Martins were among at least four assistant US attorneys to question de Blasio at the Midtown office of his lawyer, Barry Berke.

The contingent arrived around 9:18 a.m. and spent about 4¹/₂ hours inside before the meeting broke up at about 1:50 p.m.

Bharara stayed in the shadows, choosing not to attend the interview, sources said.

“What they will do now is pore over every statement the mayor made — go over it with a fine-tooth comb. They will go over it looking for any contradictions,” the federal law-enforcement source said.

“Even the slightest contradiction can be used to at least build a case or to show misleading intent.”

De Blasio spent the better part of two workdays this week preparing for the interrogation with his lawyer.

On Friday, he was scheduled to head out of town for two days to attend the Democratic National Committee meeting in Atlanta.

The feds have been probing the mayor over whether he and his advisers exchanged favors to donors for contributions to his 2013 campaign and his now-shuttered nonprofit, the Campaign for One New York.

Hizzoner raised $4.3 million for CONY, which promoted his political pet projects, including from numerous individuals and firms with business interests before the city.

City Hall Press Secretary Eric Phillips said in a statement that the mayor attended the four-hour meeting “voluntarily.”

“We remain confident that at all times the mayor and his staff acted appropriately and well within the law,” he said.

“We hope our continued cooperation will help bring a swift conclusion to the US attorney’s review. In the interest of protecting the integrity of this process, we will refrain from any further comment at this time.”

De Blasio is also under separate investigation by Manhattan DA Cyrus Vance Jr. over fund-raising tied to a failed attempt to win Democratic control of the state Senate in 2014.

He was questioned by the DA’s Office in late December.

Federal investigators have recently focused their wide-ranging inquiry on de Blasio’s relationship with Moishe Indig, a landlord who co-hosted a fund-raiser for the mayor in October 2013.

At question is whether Indig — who landed on de Blasio’s “Worst Landlords” list when he was the public advocate in 2010 — was given favors in exchange for his fund-raising and support.

One of the donors being probed is Harendra Singh, a restaurant owner who has been cooperating with the feds after being slapped with bribery and tax-evasion charges.

Singh and his relatives donated more than $27,000 to de Blasio’s 2013 campaign.

The restaurateur was named to the advisory board of the Mayor’s Fund to Advance New York, and owned The Water’s Edge restaurant in Long Island City, where the de Blasio campaign forked over $2,613 on events.

The feds have subpoenaed thousands of e-mails and documents related to de Blasio’s 2013 campaign activities and CONY.

Meanwhile, state and federal grand juries have been hearing testimony related to the separate probes into the de Blasio administration’s fund-raising. De Blasio has said he did not testify before either one.

Vance’s office has been investigating whether de Blasio aides steered donations to Ulster and Putnam County state Senate campaign committees to get around campaign-donation limits during the 2014 race.

Additional reporting by Kevin Fasick, Kaja Whitehouseand Danika Fears