“‘What the heck?’” Ms. Craig remembered thinking. “That was the first inkling we had that, hey, there’s something to do with this company that we need to figure out.”

The trio began to talk to people familiar with the president’s father and his empire. Those people told them that the company was a middleman entity created by President Trump and his siblings essentially to move cash from Fred Trump’s companies to his children. After All County bought various items for Fred Trump’s buildings, like boilers and cleaning supplies, a secretary would bill the items to Fred Trump’s buildings with a 20 to 50 percent markup. The siblings would then pocket the difference.

In short, the siblings received millions in untaxed gifts from their father, skirting a 55 percent tax on gifts over a certain value that would have cut the total significantly.

“When we came to that realization, that was a big day for us,” Mr. Buettner said.

Over the next several months, the reporters would obtain tens of thousands of pages of documents, including more than 200 tax returns from Fred Trump, his companies and various Trump partnerships and trusts. (“We have a virtual mountain of spreadsheets,” Mr. Barstow said. “We should have spreadsheets for our spreadsheets.”) The trove included previously secret depositions, including one in which Robert Trump , the president’s brother, admitted that the family used the padded receipts from All County to justify higher rent increases for their tenants in rent-regulated apartments.

This year, another breakthrough came when the team matched a boiler receipt from a personal injury lawsuit Mr. Buettner found that named All County — a man was injured by the boiler in a Trump building — to a boiler receipt they obtained via a FOIA request to New York City. They found two identical purchase-order numbers, with the bill from All County to Fred Trump marking up the boiler price by 20 percent.

“That's a rare moment of reporting serendipity, right?” Mr. Buettner said. “Two pieces of paper from two separate places that combined to tell you a bigger truth.”

“It was like these two puzzle pieces came together — one from the lawsuit and the other from the FOIA request,” Ms. Craig said. “We call it the Perry Mason moment.”