– Michael Cohen’s guilty plea is opening a door to a long-closed world — Donald Trump’s business.

Many have tried, unsuccessfully, to get a look. Trump rebuffed calls during the election to release his federal tax returns. Public advocacy groups have sought those returns and failed. Even his ex-wives, and by one account his bankers, haven’t been able to get a full view of Trump’s finances.

But now that Cohen has told prosecutors that Trump directed him to pay women for their silence and was repaid by the Trump family business, U.S. and New York authorities are taking a closer look.

“Because there are tax implications to all these transactions, it even opens up Trump’s tax returns to state and federal prosecutors: The Holy Grail,” said Frank Agostino, an attorney in Hackensack, N.J., who formerly prosecuted U.S. tax cases.

Since Cohen spoke last week, it has emerged that federal authorities have granted immunity to two key witnesses. Allen Weisselberg, the longtime chief financial officer of the Trump ­Organization, is cooperating, the Wall Street Journal reported. So is David Pecker, whose American Media Inc. made “catch and kill” deals that helped keep illicit affairs out of public view. Also circling are New York authorities, whose actions would be beyond the president’s power to issue federal pardons.

Tax charges never sound sexy. Trump and his team spend more time on Twitter and TV batting back other allegations thrown their way, like a Russian conspiracy and obstruction of justice. But tax law has been a fundamental tool for authorities since the days of mobster Al Capone. This past week, tax crimes helped sink Cohen (five of eight counts) and Trump’s former campaign chairman, Paul Manafort (six of eight).

Tax crimes rarely stand alone, as the Cohen and Manafort cases demonstrated. But when prosecutors are working through often murky investigative strands, the concrete numbers found in tax forms are often a starting point. Anyone who has signed off on false tax forms may be eager to cut a deal to avoid penalties. A key witness in the Manafort case was his onetime deputy Rick Gates, who faced tax fraud charges of his own until prosecutors dropped them in a deal for Gates’ cooperation.

For Trump and his business, Cohen’s admission this week suggested a potential tax problem. The lawyer and fixer said payments to women (who said they’d had sex with Trump) were in fact illegal donations to Trump’s campaign intended to avert bad publicity.

Cohen paid $130,000 to silence porn actress Stormy Daniels. The Trump Organization later paid him $420,000 to cover the payment, Cohen’s tax liability and a bonus, according to federal prosecutors. The company accounted for it as legal expenses.

Lawyer bills, like other business expenses, are tax deductible. Campaign contributions aren’t. Authorities, whether federal or state, are certain to ask hard questions about how the business accounted for the payment, and whether it was a legitimate expense.

They are also likely to look back at any similar deals and how Trump executives may have accounted for those transactions.

Trump probably hasn’t filed 2017 taxes yet. White House press secretary Sarah Huckabee Sanders said earlier this year he would do so by Oct. 15. The president’s personal return will reflect income from the web of limited liability companies that comprise the Trump Organization.

If any business expenses were mischaracterized, the companies could be deducting a coverup “and making the U.S. public pay for their coverup and campaign with our tax dollars,” said Agostino.

The president and his lawyers have given conflicting accounts about the payments to Daniels and Cohen and haven’t addressed any potential tax implications. In an interview with Fox News on Thursday, Trump expressed disdain for the campaign finance charges to which his former lawyer pleaded guilty. “What Michael pled to weren’t crimes,” he said.

Weisselberg’s immunity deal is a clear threat to Trump because of the executive’s extensive knowledge of the inner workings of Trump’s company, said Harry Sandick, a former federal prosecutor who is now a white-collar criminal defense attorney.

Weisselberg “knows a lot of detail about how the Trump Organization worked, not just with respect to the campaign, but in all regards and for many years,” Sandick said. “If he has immunity, he can be compelled to speak about Trump, his family and his associates.”