President Donald Trump answers questions from the news media in the Oval Office of the White House in Washington, September 11, 2019.

The Manhattan District Attorney's Office has sent President Donald Trump's accounting firm a grand jury subpoena demanding eight years of Trump's personal and corporate income tax returns as part of an ongoing investigation.

The subpoena was sent last month to the firm Mazars USA by DA Cyrus Vance Jr.'s office.

Vance is investigating issues related to hush money payoffs to porn star Stormy Daniels and Playboy model Karen McDougal, which were facilitated by Trump's former personal lawyer and fixer, Michael Cohen shortly before the 2016 presidential election.

Both women claim to have had sex with Trump years before the election. Trump has denied their allegations.

Vance is probing whether the Trump Organization falsified business records related to payoffs.

It is not clear if the DA has obtained any of the tax returns he is seeking from Mazars, or whether Vance is now investigating issues beyond the hush money payments.

Trump has refused to publicly release his tax returns despite a long-standing practice by past presidents who did so.

The New York Times first reported the news of the subpoena. A person with direct knowledge of the matter soon after confirmed the subpoena was issued to NBC News.

Marc Mukasey, a lawyer for the Trump Organization, in an email to CNBC, said, "We are evaluating and will respond as appropriate."

Mazars USA, in an emailed statement, said, "Mazars USA will respect the legal process and fully comply with its legal obligations."

"We believe strongly in the ethical and professional rules and regulations that govern our industry, our work and our client interactions. As a matter of firm policy and professional rules we do not comment on the work we conduct for our clients," the firm said.