The grand jury is set to sit for three days a week for six months, according to the newspaper, and will likely hear matter beyond just the Trump Organization probe during that time.
According to CNN, prosecutors are combing through millions of pages of documents, including Trump's tax returns.
The district attorney obtained the sought-after files earlier this year, and the office's probe continued to heat up, with the hiring of a forensic accountant and outside counsel with experience prosecuting the mob to help out on the investigation.
The DA's investigation is being carried out alongside the office New York Attorney General Letitia James, which revealed last week that the investigation into the Trump Organization was being conducted "in a criminal capacity".
The former president went on to say, "This is purely political, and an affront to the nearly 75 million voters who supported me in the Presidential Election, and it's being driven by highly partisan Democrat prosecutors".
Trump issued a statement in reaction to the news, blasting the purported move as "a continuation of the greatest Witch Hunt in American history".
Last week, Letitia James, the top prosecutor in NY state, said her inquiry into the Trump Organization was now a criminal probe. She doesn't say why her office expanded the investigation into criminal investigations.
Trump's ex-personal lawyer Michael Cohen ― jailed for tax evasion and violating campaign finance laws ― was one of his closest henchmen before turning against his former boss and deciding to cooperate with prosecutors. Next month's election is nearly certain to decide who it will be.
In a case as complex as the Trump probe, however, Tobias argued Vance's success would be contingent on prosecutors making comprehensive presentations. He argues that the investigation is part of a democratic conspiracy to silence his voters and prevent him from running for president again.
In February, the US Supreme Court upheld Vence's investigation by clarifying how prosecutors would execute and obtain subpoenas at Trump's accounting firm.
The documents are protected by grand jury secrecy rules and are not expected to be made public.
Those familiar with this issue were not allowed to speak publicly and did so on condition of anonymity.The news was first reported Washington post. His former daughter-in-law, Jen Weisserberg, is cooperating with both inquiries.
Allen Weisselberg was subpoenaed in James' civil investigation and testified twice previous year.
Jennifer Weisselberg was not available for comment.