One of the most senior officials in the Trump Organization has testified before a special grand jury empaneled by the Manhattan District Attorney's office to hear evidence against former President Donald Trump and his company, sources with direct knowledge of the matter told ABC News.

Jeff McConney is among a number of witnesses that have already appeared before the special grand jury that will decide whether criminal charges are warranted against the former president, his company or any of its employees, the sources said.

McConney, who serves as a senior vice president and controller for the Trump Organization, is the first employee of the former president's company called to testify, the sources said, and his testimony is a sign that prosecutors have burrowed deep into the company's finances.

MORE: Manhattan district attorney convenes special grand jury in Trump probe: Sources

"Complex accounting issues are crucial to this investigation, as is the knowledge and intent of the people at the Trump Organization involved in these transactions," said Daniel R. Alonso, the former chief assistant district attorney in Manhattan and now a partner in private practice at Buckley LLP.

"In any case like that, the two most important people -- whether as targets or witnesses -- are the company's CFO and the company's controller," Alonso told ABC News.

McConney was mentioned by Trump in his 2004 book, "TRUMP: Think Like a Billionaire: Everything You Need to Know About Success, Real Estate, and Life." In a chapter titled "How to Stay on Top of Your Finances," Trump describes an interaction he says he had with McConney in the late 1980s in which Trump implored McConney to always question invoices and never accept a contractor's first bid.

"Jeff got the message," Trump wrote, "and is doing a terrific job. He looks out for my bottom line as if the money were his own."

As part of his probe, Manhattan District Attorney Cy Vance has also been investigating the Trump Organization's chief financial officer Allen Weisselberg's financial dealings -- specifically, what fringe benefits he received from the Trumps in addition to his salary, and whether taxes were appropriately paid for any such compensation, sources have previously told ABC News.

"If, as has been reported, the DA is targeting Allen Weisselberg, it's a logical step to seek testimony from the controller, who presumably reports to him and works with him every day," Alonso said.

A spokesman for Vance declined to comment on the development, but ABC News has previously reported that Vance has sought to flip Weisselberg into a cooperating witness against Trump and the company.

MORE: New York attorney general's probe of Trump Organization is now also a criminal investigation

Weisselberg's former daughter-in-law, Jennifer Weisselberg, has been interviewed by the district attorney's office, she told ABC News, and was asked about topics ranging from school tuition and cars to the family apartment she lived in that the Trump Organization allegedly paid for.

"Some of the questions that they were asking were regarding Allen's compensation at the apartment at Trump Place on Riverside Boulevard," Jennifer Weisselberg told ABC News in an interview last month.

A spokesperson for the Trump Organization did not respond to a request for comment from ABC News.

Vance began investigating Trump's business practices based on the congressional testimony of Michael Cohen, who served as the former president's personal attorney and fixer.

MORE: Trump CFO faces second criminal inquiry: Sources

One focus of the investigation includes whether Trump inflated the value of certain properties to obtain bank loans and deflated the value of those same properties to pay lower taxes, sources have told ABC News.

Vance has twice fought all the way to the U.S. Supreme Court to gain access to eight years' worth of Trump's personal and business tax returns.

"This is a continuation of the greatest Witch Hunt in American history," Trump said in a statement reacting to news of the special grand jury last month. "This is purely political, and an affront to the almost 75 million voters who supported me in the Presidential Election, and it's being driven by highly partisan Democrat prosecutors."