Attorney General Eric T. Schneiderman of New York has begun investigating contributions to tax-exempt groups that are heavily involved in political campaigns, focusing on a case involving the U.S. Chamber of Commerce, which has been one of the largest outside groups seeking to influence recent elections but is not required to disclose its donors.

Mr. Schneiderman issued a wide-ranging subpoena on Tuesday to executives at a foundation affiliated with the chamber, seeking e-mails, bank records and other documents to determine whether the foundation illegally funneled $18 million to the chamber for political and lobbying activities, according to people with knowledge of the investigation.

The investigation is also looking at connections between the chamber’s foundation, the National Chamber Foundation, and another philanthropy, the Starr Foundation, which made large grants to the chamber foundation in 2003 and 2004. During the same period, the National Chamber Foundation lent the chamber $18 million, most of it for what was described as a capital campaign.

In a complaint filed last year with the attorney general, watchdog groups asserted that the loan had been used to finance lobbying for “tort reform” legislation in Congress and to run issue advertising in the 2004 presidential and Congressional campaigns, most of it against Democrats.