The FBI is investigating suspected NYPD corruption focusing on the relationship between two politically connected businessmen and a slew of officers throughout the ranks, multiple sources told The Post on Monday.

The feds are grilling about 20 cops — including three deputy chiefs and the head of the Upper East Side’s 19th Precinct — over gifts and foreign trips that the businessmen may have doled out to them in exchange for favors, law enforcement sources said.

A grand jury also has been convened, sources said.

The investigation began with an unrelated deal-gone-awry involving the two businessmen — Mayor Bill de Blasio buddies Jona Rechnitz and Jeremy Reichberg, well-placed sources said.

They gave a large sum of money to a third party who was supposed to buy discounted liquor for them to sell at a profit, but investors lost their money and the feds opened a fraud probe, sources said.

Wiretaps on the two businessmen’s phones revealed relationships with several NYPD cops — and the probe soon led the feds to then-NYPD Chief of Department Philip Banks and his close pal, corrections union president Norman Seabrook, sources said.

Reichberg, who lives in Borough Park, is a police buff known for his NYPD connections, while Rechnitz is a deep-pocketed de Blasio donor from the Upper West Side, sources said.

The businessmen became close friends with Banks and Seabrook, with the foursome traveling together to the Caribbean and to Israel in 2014.

Rechnitz picked up both men’s tab for the Caribbean jaunt, sources said. He also paid Seabrook’s airfare and hotel bills in Israel, while Banks paid for the flight but allowed Rechnitz to pick up his lodging tab, sources said.

Although he was on his personal time, Banks wore his NYPD uniform during a visit to the Western Wall in Jerusalem, and when he returned to New York, shared what he had learned with authorities here, sources said.

As a thank-you gift for all Rechnitz had done, Banks and Seabrook bought him a pricey backgammon set made of ancient wood.

“[We] spent $5,000 on the backgammon set — custom-made from Israel — so that nobody could say they bought me the [plane] ticket,’’ Seabrook told The Post Monday.

“There is no quid pro quo. There’s nothing [the FBI] could say Norman did wrong,’’ he added, referring to himself.

Banks could not be reached for comment.

Rechnitz also paid for at least part of other cops’ trips to places such as London, Rome, Las Vegas and the Caribbean, sources said.

Several sources said those being questioned as part of the probe include Deputy Inspector James Grant, head of the Upper East Side’s 19th Precinct; Brooklyn South Deputy Chief Eric Rodriguez; and Deputy Housing Chief Michael Harrington. Harrington worked for Banks before the chief of department retired.

“Most of the bosses questioned are not targets,’’ a law enforcement source claimed.

Captains Endowment Association president Roy Richter said, “A number of my members have been interviewed by federal investigators. All of them fully cooperated and were told they were not the targets or subject of the investigation.’’

One of the officers who has already testified before the grand jury is veteran NYPD Community Affairs Officer Michael Milici, sources said. The detective pleaded the Fifth and was placed on modified duty, sources said.

Rechnitz and Reichberg both served on de Blasio’s “inauguration team’’ that planned his party when he took office in 2014.

“I have done nothing wrong and have not engaged in any inappropriate behavior with anyone connected to the NYPD,’’ Rechnitz told The Post. “Beyond that, I don’t have any comment.’’

Reichberg did not return calls.

Additional reporting by Kate Sheehy, Carl Campanile and Tina Moore