Michael Cohen’s secret tapes were already causing tremors in President Trump’s world. On Thursday, the tremors might've transformed into a small quake.

Allen Weisselberg, a senior executive who spent decades at Trump’s side and is believed to know the inner workings of Trump’s businesses, has been subpoenaed to testify before a federal grand jury in the criminal probe into Cohen, The Wall Street Journal reported Thursday.

The news comes two days after Weisselberg was name-dropped in a conversation Cohen secretly recorded with Trump about buying the rights to a former Playboy model’s story about her alleged affair with Trump.

“I’ve spoken to Allen Weisselberg about how to set the whole thing up,” Cohen, Trump’s longtime fixer and personal attorney, is heard saying.

While it’s not clear when the subpoena came down, the news appears to tie one of Trump’s top business lieutenants into the Southern District of New York’s probe into Cohen, and that could spell more trouble for Trump.

Few are reputed to know as much about Trump’s businesses’ inner workings as Weisselberg, chief financial officer at the Trump Organization and an executive who’s helped run Trump’s (and his father's) business empire for decades.

That makes Weisselberg’s appearance in the probe especially concerning for the president, according to Timothy O’Brien, author of "TrumpNation: The Art of Being The Donald."

“Weisselberg isn’t a bit player in Trumplandia, and his emergence on the Cohen-Trump recording – as someone possibly facilitating a scheme apparently meant to disguise a payoff – should worry the president,” O’Brien wrote Wednesday in a column on Bloomberg. “Weisselberg has detailed information about the Trump Organization’s operations, business deals and finances. If he winds up in investigators’ crosshairs for secreting payoffs, he could potentially provide much more damaging information to prosecutors than Cohen ever could about the president’s dealmaking.”

Weisselberg’s detailed knowledge comes from working in Trump’s family-business since the 1970s. He started under Trump’s father, Fred Trump, before becoming controller of the organization in the 1980s, then Vice President of Trump Hotels & Casino Resorts. He even appeared as a judge on the second season of Trump’s reality TV show, The Apprentice. When Trump became president, he handed over control of his business holdings to his two adult sons, Eric and Don Jr., and Weisselberg.

Katy Tur, a correspondent for NBC News, tweeted that a former employee of the Trump Organization had told her that Weisselberg knew “where all financial bodies are buried within the Trump organization.”

Last summer, Trump told The New York Times that any investigation into his business activities would be a red line for special counsel Robert Mueller, who is leading a probe of Russia’s alleged interference in the 2016 campaign.

Cohen is being investigated by the U.S. Attorney’s office for the Southern District of New York. In April, that office and the FBI executed a search of Cohen’s properties that his lawyer at the time said was ordered “in part” based on a referral from Mueller’s team.

The Journal reported that Weisselberg is considered a witness in the investigation into Cohen, and that it’s not clear whether he has already appeared before the grand jury or what questions prosecutors of New York’s Southern District might want to ask him. The date of the subpoena couldn’t be determined, the paper said.