“No one, no matter how rich or famous they are, has a right to scam hardworking New Yorkers,” Mr. Schneiderman said in the statement. “Anyone who does should expect to be held accountable.”

The inquiry into Trump University came to light in May 2011 after dozens of people had complained to the authorities in New York, Texas, Florida and Illinois about the institution, which attracted prospective students with the promise of a free 90-minute seminar about real estate investing that, according to the lawsuit, “served as a sales pitch for a three-day seminar costing $1,495.” This three-day seminar was itself “an upsell,” the lawsuit said, for increasingly costly “Trump Elite” packages that included so-called personal mentorship programs at $35,000 a course.

On Saturday evening, Michael Cohen, a lawyer for Mr. Trump, denied the accusations in the lawsuit and said the school had received 11,000 evaluations, 98 percent of which rated students as “extremely satisfied.”

George Sorial, another lawyer for Mr. Trump, called the lawsuit politically motivated. He said that Mr. Schneiderman had asked Mr. Trump and his family for campaign contributions and grew angry when denied.

“This is tantamount to extortion,” Mr. Sorial said.

Andrew Friedman, a spokesman for the attorney general’s office, said that although Mr. Schneiderman had accepted a contribution from Mr. Trump in the past, “the fact that he’s still brave enough to follow the investigation wherever it may lead speaks to Mr. Schneiderman’s character.”