The Association of National Advertisers said its probe of the advertising business found that rebates and other nontransparent practices are “pervasive” in the U.S., raising alarms about how ad agencies conduct business throughout the industry.

The trade group, which represents big advertisers such as AT&T Inc., General Motors Co. and Procter & Gamble Co., said that advertising companies are being rewarded with cash rebates from media companies for spending a certain amount on behalf of clients.

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