Federal prosecutors have begun a wide-ranging effort to curb the growing popularity of online gambling in the United States by quietly threatening legal action against American companies that do business with Internet casinos and sports betting operations based outside the country, lawyers and industry executives say.

The investigation into the activities of media, public relations and technology companies relies on a controversial legal concept that holds that the American businesses, by providing advertising and other services that support Internet gambling, are “aiding and abetting” online casinos. That gives prosecutors an indirect way to attack the overseas enterprises, whose operations are illegal here but fall outside their jurisdiction.

Lawyers said they were not aware of any charges that had been filed. Still, the campaign, which has gone on for months, has already chalked up some significant nonlegal victories. Several big media operations — including Infinity Broadcasting, Clear Channel Communications and the Discovery Networks — stopped running advertisements for offshore Internet casinos last fall in light of the threat of further scrutiny that might lead to prosecution.

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