Allen Weisselberg is an unassuming, soft-spoken guy who has spent decades avoiding the limelight, first in the 1970s as an accountant for President Donald Trump’s father, Fred, and then as the Trump Organization’s chief financial officer.

Those days are over.

The Wall Street Journal reported on Friday that Weisselberg — who has also watched over the president’s personal finances and tax returns while currently running the Trump Organization with Trump’s two eldest sons — received immunity from federal prosecutors in their case against Trump’s lawyer and self-described fixer, Michael Cohen.

Weisselberg’s cooperation is a potentially momentous turn of events for the president. Depending on how prosecutors proceed, it may take the federal tax- and bank-fraud investigation of Cohen — and, more important, Special Counsel Robert Mueller’s probe involving Russian interference in the 2016 election — out of some of the relatively low-stakes legal issues in play so far and into the heart of the Trump Organization and the president’s business and financial dealings.

Last month, it was reported that a grand jury had subpoenaed Weisselberg to testify in the federal investigation of Cohen and Trump’s hush-money payments to two Trump paramours during the 2016 presidential campaign. Cohen also mentioned Weisselberg during a tape recording he made of Trump and him discussing a payment to one of the women. Immunity liberates Weisselberg from potential criminal exposure of his own to share more detailed information with prosecutors than he otherwise might have, and heightens Trump’s own legal peril.

Federal prosecutors have also granted immunity to David Pecker, who publishes the National Enquirer. In 2016, the Enquirer purchased the rights to a former Playboy Playmate’s account of an affair with Trump. Pecker has reportedly shared details about Cohen’s payments and Trump’s knowledge of the deals. Cohen pleaded guilty to campaign-finance fraud on Tuesday and said that Trump directed him to pay the hush money, making Trump criminally liable as Cohen’s accomplice.

Pecker’s immunity agreement may, therefore, be relatively narrow and limited to the prosecution of Cohen by the Manhattan U.S. Attorney. Weisselberg, on the other hand, surely knows things that travel beyond the borders of the Cohen investigation and into the more sprawling landscape of potential crimes that Mueller is investigating — including obstruction of justice, aiding and abetting a fraud, conspiracy against the U.S., and, in the words of the Constitution’s impeachment clause, possible “high crimes and misdemeanors” in the Oval Office.

To be sure, the only people who have a complete handle on the parameters of all of this are Mueller and his tight-lipped team of investigators. So it will take time for all of this to be subjected to a more transparent appraisal. Still, Trump has had few loyalists in his operation historically, and Weisselberg is certainly one of the most longstanding members of that tiny club. His cooperation with investigators will resonate personally for the president, and is likely to force an already combative man to lash out in ever more forceful ways.

Weisselberg is deeply familiar with the Trump Organization’s financial housekeeping. Trump — a man who rarely trusts anyone — confided in Weisselberg and relied on him to sign off on details of the company’s most significant deals. Weisselberg oversees the trust that Trump set up to manage his interests in the Trump Organization while he’s in the White House, and also had a prominent position inside the president’s troubled charitable foundation. In short, he was privy to decisions at the Trump Organization that Cohen was never allowed to take part in.

That kind of knowledge is gold to federal investigators. Mueller’s team signaled long ago that it might take a closer look at the president’s business dealings as part of its examination of Russia’s assault on the presidential campaign. It’s likely that the probe is exploring whether Trump or others on his business and campaign teams — including members of his family — discussed exchanging policy favors (lifting economic sanctions on Russia, for example, or shifting the U.S. stance on Russia’s military incursion into Ukraine) in exchange for financial or political quid pro quos.

Trump’s intersection with murky funding from overseas sources, including Russia, goes back years (as I’ve detailed before in columns about the Trump SoHo hotel and Trump’s partnership with the Bayrock Group). In a column about Cohen and Weisselberg in April, I noted that Weisselberg was a possible candidate for a subpoena given the fact that Mueller had already subpoenaed the Trump Organization for business records — and given the fact that Weisselberg has had a front row seat in deals involving transactions like the Trump SoHo.

Adding all of that up suggests that federal prosecutors have had incentives to pressure Weisselberg to reconsider his loyalties to Trump. That makes the Trump family’s reclusive accountant a significant player in this unfolding historical drama, even though he spent decades shunning the spotlight his famous boss courted so obsessively. And remember: It was a forensic accountant — the guy who knew all the boring details about taxes and buried financial bodies — who ultimately brought down Al Capone.

Bloomberg News