Schneiderman, a supporter of Trump's Democratic rival in the presidential race, Hillary Clinton, sued the real estate mogul and his now defunct Trump University for fraud in 2013, seeking $US40 million ($A54 million) in restitution plus penalties and other costs.
In a CNN interview on Tuesday, Schneiderman said his office had now brought Trump's charitable foundation under scrutiny.
"My interest in this issue really is in my capacity as regulator of nonprofits in New York state. And we have been concerned that the Trump Foundation may have engaged in some impropriety from that point of view," the elected Democratic official said.
"We have been looking into the Trump Foundation to make sure it's complying with the laws that govern charities in New York."
He did not elaborate on what wrongdoing Trump's nonprofit might have committed.
Trump campaign spokesman Steven Cheung dismissed Schneiderman's assertions as entirely motivated by presidential politics, calling the attorney general a "partisan hack who has turned a blind eye to the Clinton Foundation for years."
Cheung called Schneiderman's comments "nothing more than another left-wing hit job designed to distract" attention from Clinton's recent missteps on the campaign trail.
Trump has accused Clinton of being corrupted by donors to the Clinton Foundation global charity founded by her husband, former President Bill Clinton, while she was US secretary of state from 2009 to 2013.
Clinton has dismissed Trump's allegation as a political smear.
There has been no evidence that foreign donors to the foundation obtained favours from the State Department while Clinton headed the agency.
