The office has been investigating possible civil violations by the Trump Organization over the way the company valued holdings as it sought loans and tax benefits.
Now the investigation has entered a "criminal capacity," the spokesperson said.
"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," said Fabien Levy, a spokesman for New York Attorney General Letitia James.
The New York attorney general notified the Trump Organization of the change in recent months, a source familiar with the matter told ABC News.
It's not immediately clear what practical effect the notification has, because any potential criminal charges would be brought by the Manhattan district attorney's office.
Representatives with the Manhattan DA's office declined to comment on the development.
In a separate investigation, the Manhattan district attorney's office has been presenting evidence to a grand jury and poring over Trump's personal and business tax returns as it considers whether to file criminal charges, sources tell ABC News.
Officials with The Trump Organization declined to comment on the matter.