The New York attorney general's office says it is investigating the Trump Organization "in a criminal capacity".
A spokesman for the state's top prosecutor, Letitia James, said the inquiry into Mr. Trump's property company was "no longer purely civil".
Ms. James has been scrutinizing the ex-Republican president's financial dealings before he took office.
The Trumps deny wrongdoing and say the inquiry by a Democratic prosecutor is a political vendetta.
Ms. James' spokesman, Fabien Levy, told the BBC on Tuesday: "We have informed the Trump Organization that our investigation into the Organization is no longer purely civil in nature.
"We are now actively investigating the Trump Organization in a criminal capacity, along with the Manhattan DA. We have no additional comment."
The statement did not say what turned the inquiry from civil to criminal in nature, or whether the former president himself might be personally implicated in any allegations.
