In late 2016, I had lunch with a former high-ranking Trump Organization executive, a person who said he was happy to share dirt on his old boss, but who confessed to not having much dirt to share. This executive wrote a list of people whom I might contact to find out about anything potentially illegal or unethical that Donald Trump may have done. At the bottom of the list was the name Weisselberg. “Allen is the one guy who knows everything,” the person told me. “He’ll never talk to you.” I have had nearly identical conversations with different people who work or have worked for the Trump Organization many times since. They all described his role similarly: Allen Weisselberg, the firm’s longtime chief financial officer, is the center, the person in the company who knows more than anyone.

On Friday, the Wall Street Journal broke the story that Weisselberg had been granted immunity by federal prosecutors in the Southern District of New York so that he could share information in the investigation of Michael Cohen, Trump’s former personal attorney and dealmaker. It is safe to say that the entire world of Trump watchers—those journalists, political folks, and advocates who carefully monitor every bit of Trump news—went bonkers. Weisselberg is the man to whom those people most want to speak. He is also the man who has, for decades, been the most circumspect.

As the C.F.O., Weisselberg tracked the money that came into the Trump Organization and the money that went out of it, former employees told me. I often found myself wondering what the Weisselberg part of the operation looked like. (I called and e-mailed him a few times, but, not surprisingly, never heard back.) Some told me he had a couple of bookkeepers, but that he personally handled most of the paperwork. Weisselberg knew who was paying or lending money to Trump, and he knew to whom Trump was giving money. When Trump became President, he placed his business interests in a revocable trust overseen by his son Donald Trump, Jr., and Weisselberg.

Weisselberg has been notoriously press-shy for decades. I recently conducted a search of Nexis, the news database, and found him mentioned only in a handful of articles before Trump’s Presidential campaign. Most were reviews of his staccato performance on the TV show “The Apprentice,” or mentions that Weisselberg declined to comment on this deal or that.

This summer, though, Weisselberg’s role in the organization came into sharper focus. In a recording that Michael Cohen made of a conversation he had with Donald Trump about a payment to keep secret an affair, Cohen described setting up a shell company to pay hush money during the 2016 campaign to Karen McDougal, a woman who claimed to have had an affair with Trump. This week, Cohen pleaded guilty to violating campaign-finance laws, in part by setting up this secretive payment. He said that he knew at the time that it was illegal to secretly make a payment for campaign-related activity, but he did so anyway at Trump’s direction. Strikingly, Cohen makes it clear on the tape that Weisselberg also knew about the shell company and payment. “I’ve spoken to Allen Weisselberg about how to set the whole thing up,” Cohen explains to Trump.

It is difficult to hear the tape and not wonder how Weisselberg developed this particular expertise and whether he had deployed it before. More importantly, it offers more justification for Robert Mueller and other federal, state, and local prosecutors to investigate the Trump Organization’s general business practices. The tape suggests that Weisselberg provided guidance for Cohen’s payment, an illegal act designed to influence the election. Since one of the Mueller investigation’s core aims is to understand whether the Trump campaign worked with Russia to sway the 2016 Presidential election, Weisselberg now seems to be a key witness. Worse, for Trump, if Weisselberg, fearing prosecution himself, tells prosecutors of other criminal activity in the organization, that information will likely be referred to other federal and state prosecutors, thus broadening the investigation of Trump’s business.

Weisselberg has worked exclusively for the Trump family for his entire adult life. Shortly after graduating from college, he began working for Fred Trump, in 1970, and then for the young Donald when he entered the business. Weisselberg’s son Barry works at the Trump-run Wollman Skating Rink, in Central Park; his other son, Jack, works at Ladder Capital, which has been a primary lender to the Trump Organization in recent years, when few other lenders would work with a company that had experienced several bankruptcies.

Last month, the New York State Attorney General, Barbara Underwood, sued the Trump Foundation. Weisselberg had been deposed and showed a surprising willingness to give answers that put the President in an unflattering light. In January, 2016, during Trump’s Presidential campaign, his foundation made a series of donations to veterans-advocacy organizations in Iowa that were explicitly designed to gain support for his candidacy. Weisselberg filled out the checks. In his deposition, he volunteered that the Trump Foundation had no procedures in place to insure it followed the law and that Trump himself knew of and directed Weisselberg’s participation in the scheme to pay those Iowa veterans groups. Were Weisselberg eager to protect his longtime boss, he could have answered the questions far more narrowly. It was an early hint that Weisselberg, like Cohen, may not jeopardize his own freedom to defend Trump. News that Weisselberg had accepted immunity so that he could share potentially damaging information in the Cohen case provides more support for the view that Weisselberg is ready to share whatever information he has. And he has a lot.

There are many open questions about how, precisely, the Trump Organization has made and spent its money in recent years. There is, for example, a question about where Trump got more than two hundred million dollars in cash to buy and lavishly upgrade a money-losing golf course in Scotland. In a deal in Azerbaijan, Trump knowingly did business with a family that is widely suspected of laundering money for Iran’s Revolutionary Guard. The F.B.I. has reportedly investigated the source of funds for a Trump-branded property in Vancouver, Canada; while the Trump hotel in Toronto also has suspicious funding. These are just a handful of the many business deals that Trump has conducted that have signs of possible money-laundering, tax evasion, sanctions violations, and other financial crimes. Many of the key questions about Donald Trump revolve around his funding sources and his business partners: Did he knowingly receive funds from criminals? Did he launder money for criminals? Did he receive remuneration to look the other way when his partners broke the law? Was much of his business built around selling his famous name to make illegitimate projects seem viable? More broadly, where did his money come from? Where did his money go? And how much questionable activity has he hidden from the world? Trump, himself, may not know the exact answers to all of these questions. Perhaps Allen Weisselberg does.

The Journal story and other news coverage suggest that Weisselberg has narrow immunity, related, solely, to the payments that Michael Cohen made to silence two women with whom Trump had affairs. With evidence of that crime in hand, prosecutors can subpoena other records from the company. If they have a reasonable basis to believe another crime has been committed, they can ask Weisselberg about it. Weisselberg, fearing jail time himself, could broaden his coöperation. The fact that Weisselberg has “flipped”— and may flip further—could shift the calculus of other figures in the Trump orbit as well. Weisselberg is a big fish—perhaps the biggest fish of all. Fearing that Weisselberg might implicate them in a crime, any cronies, dealmakers, attorneys, and others who might want to exchange information for leniency from prosecutors, will now do so.

There are now multiple investigations of the Trump Organization being conducted by the special counsel Robert Mueller, the New York Attorney General, The New York State Department of Taxation and Finance, the Manhattan District Attorney, the Southern District of New York, and—quite likely—other jurisdictions. President Trump is unable to stop most of these investigations. With Cohen and, now, Weisselberg providing information, it is becoming increasingly certain that the American people will—sooner or later—have a far fuller understanding of how Donald Trump conducted business. That is unlikely to go well for him.