For the first time, he was operating without the safety net of his late father Fred’s real estate empire, which had been sold off in 2004, according to a New York Times investigation published last week.

That means that between 2005 and 2015, when Trump expanded his hotel chain from three locations to 12 and increased his golf courses from four to 15, he was finally on his own.

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“Fred was a piggy bank that Trump could routinely go to when he needed a cash infusion. [But then] the piggy bank disappeared,” said Tim O’Brien, a journalist who researched Trump’s business extensively for his 2005 book “TrumpNation.”

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Trump received $177.3 million from the sale of his father’s remaining holdings, of which he quickly used $149 million for pressing needs at his own ventures, according to the Times. After that, he bought 14 properties with cash alone, without taking on loans, in a $400 million spending spree that defied industry norms, as The Washington Post previously reported. To buy other properties, he got more than $300 million in loans from an unusual source — the private-wealth management office of Deutsche Bank, according to public documents. And Trump ended up with a loan of more than $50 million that he still owes himself, according to financial disclosures.

The Trump Organization — the private company that President Trump still owns but does not manage — did not respond to requests for comment. The White House did not respond to a request for comment.

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Company officials have rejected the idea that it needed outside financial help during this period of expansion. Trump’s son Eric, who now helps run the Trump Organization, said his father’s businesses produced so much cash that there was plenty to make large purchases.

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“He had incredible cash flow and built incredible wealth,” Eric Trump told The Post earlier this year. “He didn’t need to think about borrowing for every transaction. We invested in ourselves.”

Trump’s unusual financial tactics — about which the Trump Organization has provided few details — are poised to come under scrutiny from Congress next year if Democrats take over one or both chambers.

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“The only way to check on this is to get his tax returns,” said Rep. Bill Pascrell Jr. (D-N.J.), a member of the tax-writing House Ways and Means Committee.

If Democrats take control of the House in this fall’s midterm elections, Pascrell said, the committee will seek to obtain his returns to scrutinize Trump’s business partners and creditors.

Meanwhile, tax officials in New York state are assessing whether to open an investigation into the tax practices detailed in the Times story. And New York Mayor Bill de Blasio (D) said last week that the city government “is looking to recoup any money that Donald Trump owes the people of New York City, period,” according to news reports.

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The Times, relying on hundreds of thousands of documents from Fred Trump’s business empire, reported that the New York developer had funneled at least $413 million to his son over his lifetime, using what the paper called “dubious tax schemes” to avoid taxes on gifts.

The Times said that those payments had helped Donald Trump’s business survive crises of its own making. The most dramatic example came in 1990, when one of Donald Trump’s Atlantic City casinos was struggling to make its payments. Fred Trump bought $3.5 million in casino chips and then didn’t use them, effectively giving his son’s casino an interest-free loan, the Times reported.

The details contradict Donald Trump’s claim that his father “gave me a very small loan in 1975,” as he put it during a 2016 presidential debate. At other times, Trump has said the amount of the loan was $1 million.

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In a statement to the Times, Trump attorney Charles Harder said that its findings were “100 percent false, and highly defamatory.”

“There was no fraud or tax evasion by anyone,” Harder told the newspaper. “The facts upon which The Times bases its false allegations are extremely inaccurate.”

Some details in the report surprised even Trump’s biographers, who had spent years trying to understand the family’s finances.

“It never really kind of added up,” how Trump had escaped financial trouble so many times, said Gwenda Blair, who wrote a 2001 history of the family called “The Trumps.” The answer, she said: “There was dad. The Bank of Dad.”

Fred Trump died in 1999. In 2004, the Times said, Donald Trump induced his siblings to sell their father’s remaining apartment buildings for $737.9 million — giving up a steady stream of income for a one-time infusion of cash. Trump used his share to buy out a partner in a Chicago hotel, to buy a house in Florida and to pay off casino creditors in New Jersey, the Times reported.

Around that time, Trump found a new source of cash — selling his name, parlaying fame from his reality-TV show “The Apprentice” into deals to put the Trump brand on everything from hotels to cologne.

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Last year, Yahoo Finance — citing internal documents from the show — said “The Apprentice” made Trump at least $65 million between 2004 and 2007.

By 2015, Trump was earning more than $2.4 million annually by merchandising products with his name — including a Trump-branded urine test, according to Trump’s financial disclosures. He was also paid to put his name on other people’s projects, including a hotel in Vancouver, B.C., and golf courses in Dubai.

At the same time that Trump’s income from his TV career was growing, he was facing challenges in other parts of his business. His Atlantic City casino company filed for bankruptcy in 2004, facing $1.8 billion in debt. A few years later, the financial crisis hurt the value of his newer holdings, such as the Chicago tower where he was trying to sell condos. Then Trump actually sued his own lender — the commercial-lending arm of Deutsche Bank — in an attempt to avoid a loan payment in 2008, according to legal documents.

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Nevertheless, around the same time, Trump had enough cash to start buying up real estate in an un­or­tho­dox way — without borrowing money through a mortgage.

These all-cash transactions began relatively small. In 2006, Trump paid $12.6 million for a plot of land in Scotland where he later developed a golf course. The deals grew over time, culminating with the $67.8 million purchase of another Scottish course, Turnberry, in 2014.

In all, these all-cash purchases — five houses, eight golf courses and a winery — cost Trump more than $400 million, according to an analysis by The Post.

They defied an axiom of the real estate industry — that borrowing some money is smarter than spending only one’s own cash, because it leaves the developer with less at risk.

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And they defied Trump’s own history. For years he had bragged that he was “the king of debt” and that he loved investing with other people’s money.

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Eric Trump said there was no secret to this change in his father’s buying habits: He had the money to buy, so he did. “It’s a very nice luxury to have,” Eric Trump said earlier this year.

Donald Trump also found financing for some projects from an unconventional source for a developer.

In 2012, at the tail end of the Chicago project, Trump received more than $300 million in loans from the private banking arm at Deutsche, a German bank, according to public records.

A private banking office is typically dedicated to managing the finances of wealthy individuals, not to issuing large commercial construction loans.

“It’s highly unusual to go through the private bank,” said Jeffrey M. Zell, a D.C. real estate consultant. Among a borrower’s reason for doing so, he said, would be to avoid the underwriting criteria or collateral requirements normally employed by commercial bankers who underwrite construction.

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“The bank’s construction guys will go out and check the contractors’ bids, check the borrowers’ reserves,” he said. “The private bank will say, ‘We don’t care, as long as the borrower has cash.’ ”

The loans that Trump’s company received from this office of Deutsche Bank helped pay off the last of a Chicago loan that once totaled $640 million and provided Trump with $125 million to renovate the Doral golf resort in Miami and $170 million to develop the Trump hotel in Washington.

During the 2016 campaign, Trump cited an executive in the private banking division, Rosemary Vrablic, as an expert on his finances.

Deutsche Bank declined to comment on its relationship with Trump.

Some of Deutsche’s lending practices have drawn scrutiny from regulators in the United States and abroad. Last year, Deutsche agreed to pay $630 million in fines to American and British regulators for “mirror trades” that regulators said allowed Russian investors to launder $10 billion through the bank over four years.

Trump emerged from his decade-long buying spree with another major debt — that oddly, according to records, he owes himself.

Since 2012, according to his financial disclosures, Trump has owed more than $50 million to a company called Chicago Unit Acquisition LLC. The disclosures indicate that the debt is related to the Trump hotel in Chicago but do not require him to reveal the exact amount.

Chicago Unit Acquisition isn’t a bank. It doesn’t have its own office. Instead, it is headquartered at Trump Tower in New York — and owned by Donald Trump.

So why would Trump owe himself more than $50 million?

The Trump Organization has declined to answer questions from The Post about the loan.

But in 2016, Trump told the Times that the loan began as a debt he owed to a group of lenders. Instead of paying it back, however, he bought the loan itself — and kept it on his own books, as a debt one part of his empire owed to the other.

“I have the mortgage. That is all there is. Very simple. I am the bank,” he told the Times.

He did not explain what loan he had purchased.

The Post checked both major loans that were outstanding on the Trump building in Chicago in 2012, but neither appeared to fit Trump’s description. There was a $640 million loan owed to Deutsche Bank’s commercial-lending side, but public documents show it was paid off and canceled, thanks to money Trump got from the bank’s private-wealth side.

And there was a $130 million loan Trump owed to another group of banks. Public documents show that it also ended in 2012. Two people involved in that deal said that Trump did not purchase that loan, either — he simply paid it off at a discount, paying $48 million instead of $130 million.