On a quiet street in Brooklyn’s posh Cobble Hill, neighbors of 377 Union St. have grown increasingly agitated at the owner of the building — which has sat empty and neglected for four years.

Construction materials litter the front and back yards, and the owner has done nothing to make this quiet street more livable.

The owner, the neighbors discovered some months ago, is none other than Paul Manafort, the millionaire former campaign manager for Donald Trump, who finds himself being investigated by the FBI for his connections to the Kremlin as part of the Trump campaign.

The 68-year-old politico is also being probed by New York Attorney General Eric T. Schneiderman and Manhattan DA Cyrus Vance Jr.

The two were reported to have started an investigation into Manafort’s real estate dealings earlier this month.

The Trump consigliere, who resigned from that post in August, has been at the center of the Trump-Russia investigation in part for his business dealings with pro-Russian ex-Ukraine Prime Minister Viktor Yanukovych from 2004 to 2010.

While the US and state probes have naturally dominated media reports, much less is known about Manafort’s curious history on Union Street, one of the most desirable locations in the borough.

The story of the brownstone took shape after The Post interviewed more than a dozen people connected to the property.

Most of them asked to remain anonymous because they feared that speaking about Manafort and his family would endanger their reputation or safety.

Manafort’s connection to No. 377 begins in 2012 when the three-family property in Carroll Gardens, one of the hottest neighborhoods in Brooklyn, hit the market.

The four-story Italianate brownstone came with “all the bells and whistles,” like arched marble mantles and a Jacuzzi, according to a real estate listing at the time.

While Union Street sees more car traffic than the others in the neighborhood — it’s a major thoroughfare that cuts to Eastern Parkway and Prospect Park — the block is lined with manicured gardens of blooming flowers.

At the time, No. 377 was listed for nearly $3 million — and attracted plenty of serious interest.

Two people who visited and had a look around were Paul Manafort’s daughter, Jess, a filmmaker then living in Soho, and her husband, Jeff Yohai, a real estate investor.

They came several times, but were “hardly present,” one person familiar with the visit told The Post recently.

One time, the owner’s pot-bellied pig got loose from the back garden, and Yohai ran from it yelling, “Oh my God, a pig!” as it barreled through the 113-year-old home.

In late October 2012, the building’s broker was approached by a lawyer with an offer that could hardly be refused: the full ask — and in cash.

Manafort or his lawyer had told the people involved in the sale that the house was intended as a wedding gift for the young couple, who had married on March 2, 2012, at the swanky Pratt Mansion on Manhattan’s Upper East Side.

That wasn’t the first high-end property that the future Trump campaign manager purchased.

Manafort had already bought two other properties through secretive shell companies in New York — Apartment 43-G in Trump Tower, which he bought in 2006, and, six years later, a loft on Howard Street, where Jess and Yohai were then living.

It isn’t clear why Manafort would buy them another home as a gift, since he had just bought the Howard Street property for Jess and Yohai to live in earlier that year.

What makes the transaction even more curious is the apparently strained relationship Manafort had with Yohai.

“Yohai is back to gambling. In SERIOUS debt. Bookies are after him,” Jess’ sister Andrea wrote in a text message to another person, according to messages hacked from her phone and released on the web earlier this year.

The texts haven’t previously been published.

“My dad wholeheartedly opposes her marrying Yohai right now,” she wrote in another.

The dates of the texts couldn’t be ascertained.

Further texts indicate that Paul had given Yohai at least $90,000.

“My dad no longer accepts Yohai as family,” Andrea wrote in a text.

Initially, when Jess and Yohai were first looking to buy the house, there was some talk of getting financing, but that was quickly nixed because Paul Manafort wanted to close on the deal quickly — like by the end of the year, just weeks away.

Persons close to the sale said that either Paul Manafort or his lawyer insisted on a quick closing date.

The demand was met, as the sale closed on Dec. 28, 2012, sources said.

Jess wasn’t present for the closing, and Yohai “didn’t seem to care,” one source said.

The house was purchased by a shell company, called MC Brooklyn Holdings. A costly renovation was already in the works, being carefully overseen by Jess and her husband, sources said.

Their taste ran “shabby-chic,” and the plan was almost a full-scale renovation that would keep only some of the rooms intact, sources said. A basement “speakeasy,” for example, was going to get converted to a personal gym, sources said.

Demolition began in 2013.

The contractor, Steven Jacobsen, had worked with Paul Manafort before on previous properties and came in from the Hamptons to tear down the interior.

The work would take about six months.

At some point toward the end of the demo there was a disagreement between Jacobsen and the Manaforts that led Jacobsen to no longer work on the project, sources said.

Jacobsen, through his company and a relative, declined to comment on this story.

However, a second person close to the Manaforts said the couple and Jacobsen continue to have a good relationship.

Regardless, since Jacobsen left No. 377 midway through 2013, very little work has been done on the building. It sits, the interior covered in dust, with a couple of broken windows.

Building materials, such as cinder blocks, litter the front and back yards, and the crabgrass in the front needs a mowing.

Deep cracks cut through the side of the front staircase.

Last week, a letter addressed to Paul Manafort was sticking out of the mailbox.

The costs for contractors, architects, materials, expediters and labor would cost upward of $1 million — and it was all paid for in cash, sources said.

While Yohai initially paid for the renovation, Paul Manafort ended up covering some of the costs at the end, sources said.

The fate of the building — and who will end up living there — is unclear.

Jess Manafort and her husband, who have been residents of California for years, are in the midst of getting a divorce, according to court documents relating to a civil suit filed in New York by a former business partner of Yohai claiming the developer ran a “Ponzi scheme.”

Yohai is fighting the suit.

After buying the brownstone for $3 million cash, Manafort has used it and other Big Apple properties he has paid cash for as collateral for nearly $20 million in loans, according to public filings.

Most recently, he received $6.5 million in two loans from Federal Savings Bank that use No. 377 as collateral, public filings show.

The purpose of the loan is partially to refinance about $1.5 million in old debt, according to documents.

The rest is to pay for architects and construction, a person close to Manafort said.

While one real estate expert told The Post it was unusual to borrow against more than what the house was worth, Manafort is projecting that the building will be worth more than the $6.5 million he has taken out against it.

After years of the building sitting, seemingly frozen in time, there seems to be some movement of late.

In January, the brownstone was transferred from MC Holdings to Paul Manafort.

And then this week, in response to questions from The Post, Manafort claimed he has not given up on the property.

“I have engaged an architect and contractor to complete the construction within the next year,” Manafort told The Post in a statement.

“Our permit is in place, workers have been there this week, and I am pleased to say progress is being made.”