The chief financial officer of the Trump Organization has been subpoenaed to testify before a federal grand jury investigating Donald Trump’s former personal attorney Michael Cohen, sources have told the Wall Street Journal.

Allen Weisselberg’s testimony could create major trouble not only for Cohen but also for the president. Weisselberg, 71, has a thorough knowledge of Trump deals, payments and income. The Trump Organization is currently being run by Trump’s sons Eric and Donald Jr., and Weisselberg, the Journal noted.

Weisselberg’s name emerged this week in the tape obtained by CNN of a pre-election conversation between Cohen and his boss concerning onetime Playboy model Karen McDougal, who claimed to have had an affair with Trump. In that recording, which appears to involve a payment to suppress her story, Cohen says: “I’ve spoken to Allen Weisselberg about how to set the whole thing up.” He adds later that he “spoke to Allen about it, when it comes time for the financing ... .”

Weisselberg was also aware of a $35,000-a-month retainer arrangement from a Trump trust to reimburse Cohen in 2016 after he paid $130,000 to porn actress Stormy Daniels, who also claimed she had an affair with Trump, The New York Times reported. But one source told the newspaper that Weisselberg was unaware that Cohen had paid Daniels when the retainer payments were initially set.

NEW: Loyal Trump CFO Allen Weisselberg has been summoned to testify before a grand jury in the Michael Cohen probe https://t.co/aTUxoltrPF — Yashar Ali 🐘 (@yashar) July 26, 2018

Weisselberg, who prepares Donald Trump’s taxes, according to the Journal, also served as treasurer of The Donald J. Trump Foundation, which has been sued by the New York attorney general and is under investigation for possible tax violations, and he was on the board of Trump’s Miss Universe Organization.

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“He plays an integral part in the Trump Organization’s growth and continued financial success,” Ivanka Trump said in an email to the Wall Street Journal in 2016 while the newspaper was preparing a profile of Weisselberg. “He is deeply passionate, fiercely loyal and has stood alongside my father and our family for over [three] decades.”

The Washington Post called the subpoena possibly the “biggest shoe to drop” from the Cohen tape.

Jake Tapper on CNN’s “The Lead” cited an unnamed source from inside the Trump Organization who called Weisselberg’s subpoena the “ultimate nightmare scenario” for Trump because Weisselberg “knows anything and everything.”

A top Trump Organization official and the company's longtime chief financial officer has been subpoenaed to testify, WSJ reports. This is the “ultimate nightmare scenario for Trump,” a former employee tells CNN https://t.co/Xmktk2Oy5w pic.twitter.com/UL6tHJUPKN — The Lead CNN (@TheLeadCNN) July 26, 2018

Timothy O’Brien, who wrote the Trump biography Trump Nation and interviewed Weisselberg, called Cohen a “small fish” in “Trumplandia,” while Weisselberg is a major player. Weisselberg knows “more about the Trump Organization’s history and finances than nearly anyone,” O’Brien wrote in an opinion piece for Bloomberg. His emergence on the Cohen tape “should worry the president,” O’Brien added.

An attorney for the Trump Organization, Alan Futerfas, told The Washington Post on Tuesday that it was unlikely Cohen had discussed setting up an arrangement to pay McDougal with Weisselberg, calling Weisselberg a “bookkeeper” who “simply carries out directions from others about monetary payments and transfers.”

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This article originally appeared on HuffPost.