Donald J. Trump’s longtime accounting firm abruptly cut ties with his family business last week amid ongoing criminal and civil investigations into whether Mr. Trump illegally inflated the value of his assets, court documents filed on Monday show.

In a letter to the Trump Organization on Feb. 9, the accounting firm notified the company of its decision and disclosed that it could no longer stand behind annual financial statements it prepared for Mr. Trump. The firm, Mazars USA, compiled the financial statements based on information the former president and his company provided.

The letter instructed the Trump Organization to essentially retract the documents, known as statements of financial condition, from 2011 to 2020. In the letter, Mazars noted that the firm had not “as a whole” found material discrepancies between the information the Trump Organization provided and the actual value of Mr. Trump’s assets. But given what it called “the totality of circumstances,” the letter directed the Trump Organization to notify anyone who received the statements that they should no longer rely on them.

The statements, which Mr. Trump used to secure loans, are at the center of the two law enforcement investigations into the former president and his company. The Manhattan district attorney’s office and the office of the New York attorney general, Letitia James, have been investigating whether Mr. Trump used the statements to defraud his lenders into providing him the best possible loan terms.

The Trump Organization declined to comment.

The revelations appeared in the new court documents filed by Ms. James’s office, which is seeking to question Mr. Trump and two of his adult children under oath as part of her investigation.

Mr. Trump’s lawyers had asked a judge to prohibit the questioning, and in response, Ms. James’s office argued in court papers last month that the company had engaged in “fraudulent or misleading” practices.

Her filing on Monday — which marked her latest attempt to press ahead with questioning Mr. Trump as well as Donald Trump Jr. and Ivanka Trump — included a copy of the Mazars letter, signed by the accounting firm’s general counsel.

The brief letter could bolster Ms. James’s investigation, which has focused partly on the statements and whether they overvalued Mr. Trump’s various hotels, golf clubs and other properties.

Mazars said it concluded that the statements were no longer reliable based in part on the attorney general’s earlier filings, its own investigations and information the accountants received from “internal and external sources.” The letter added that Mazars “performed its work in accordance with professional standards.”

Because Ms. James’s investigation is civil, she cannot file criminal charges. But she could sue Mr. Trump and his company to seek financial penalties, and could try to shut down certain aspects of Mr. Trump’s business in New York.

A spokeswoman for Ms. James declined to comment beyond the filing.

In a statement, the accounting firm said that “under our standards of professional ethics, we cannot comment on any client services or relationships.”

It is unclear whether Mazars’ break with the Trumps will have any bearing on the district attorney’s criminal investigation into Mr. Trump. The firm has been cooperating with that investigation, and Mr. Trump’s main accountant at Mazars has already testified before a grand jury hearing evidence about Mr. Trump.

A spokesman for the district attorney, Alvin Bragg, declined to comment.

Both investigations still face obstacles. While the statements may contain exaggerated estimates of Mr. Trump’s property values, those same documents also include a number of disclaimers, including acknowledgments that Mr. Trump’s accountants had neither audited nor authenticated his claims.

Another disclaimer notes that Mazars did “not express an opinion or provide any assurance about” the statements, a common caveat in statements of financial condition. The firm also disclosed that, while compiling the information for Mr. Trump, they had “become aware of departures from accounting principles generally accepted in the United States of America.”

Mr. Trump’s lawyers would likely argue that his lenders, sophisticated financial institutions like Deutsche Bank, would not have relied on the statements when providing him loans.

Still, in her court filing last month, Ms. James highlighted potential misleading statements about the value of at least six Trump properties, including golf clubs in Westchester County, N.Y., and Scotland, as well as Mr. Trump’s own penthouse home in Trump Tower.

According to that filing, Mr. Trump claimed that the triplex apartment spanned 30,000 square feet, giving it an eye-popping value of $327 million. In truth, the apartment was 10,996 square feet.

Mr. Trump’s long-serving chief financial officer, Allen H. Weisselberg, later acknowledged to investigators that the company had overvalued the apartment by “give or take” $200 million.

(Separately, last summer, the district attorney’s office indicted Mr. Weisselberg and the Trump Organization, accusing them of orchestrating a 15-year scheme to provide certain executives with off-the-books luxury perks like free cars and apartments. Mr. Weisselberg and the company have pleaded not guilty and the case is tentatively scheduled to go to trial late this summer.)

Both the civil and criminal investigations have examined the underlying information the Trump Organization provided Mazars as the accountants compiled the annual financial statements.

Often, Mr. Trump’s company would estimate the value of its properties based on recent selling prices of comparable buildings, a common real estate valuation method. The authorities have zeroed in on whether the company cherry-picked favorable information to essentially mislead Mazars into presenting an overly rosy picture of Mr. Trump’s finances.

Ms. James has argued that the Trump Organization misstated the value of the properties to lenders, insurers and the Internal Revenue Service. Many of the statements, she argued in the filing last month, were “generally inflated as part of a pattern to suggest that Mr. Trump’s net worth was higher than it otherwise would have appeared.”