The National Chamber Foundation then loaned $18.1 million to the U.S. Chamber of Commerce, the letter said. There was no indication that any significant part of the loan had been repaid, it said.

Schneiderman's office began the probe after receiving a complaint from a watchdog group last year, the person familiar with the situation said.

The Starr Foundation did not return phone calls seeking comment. A spokeswoman for the law firm Boies Schiller and Flexner, which represents Greenberg in a different matter with the New York Attorney General's office, did not comment on the latest probe.

Jennifer Givner, a spokeswoman for Schneiderman, declined to comment on the investigation, which was first reported by the New York Times on Wednesday.

The U.S. Chamber of Commerce is the world's largest business organization and is the voice in Washington, D.C. for more than three million businesses, according to its website.