Mark Pomerantz resigned last month, along with another senior prosecutor, Carey Dunne, after District Attorney Alvin Bragg "reached the decision not to go forward" with the criminal case, Pomerantz's resignation letter said.
The letter was first obtained by The New York Times and sources familiar with it confirmed the contents to ABC News.
"I believe that Donald Trump is guilty of numerous felony violations of the Penal Law in connection with the preparation and use of his annual Statements of Financial Condition," Pomerantz wrote. "His financial statements were false, and he has a long history of fabricating information relating to his personal finances and lying about his assets to banks, the national media, counterparties, and many others, including the American people. The team that has been investigating Mr. Trump harbors no doubt about whether he committed crimes - he did."
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Trump has denied wrongdoing. His company and his long-serving chief financial officer, Allen Weisselberg, have pleaded not guilty to charges they've faced in connection with the investigation.
Bragg has said nothing publicly about his decision whether to pursue an indictment against the former president.
His spokeswoman, Danielle Filson, said the investigation continues.
"A team of experienced prosecutors is working every day to follow the facts and the law. There is nothing more we can or should say at this juncture about an ongoing investigation," Filson said in a statement provided to ABC News.
This is breaking news. This story will be updated as more information becomes available.
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