An eight-month probe by a marketing trade group revealed that ad agencies in the U.S. are accepting rebates from media companies, said people familiar with the matter, findings likely to stoke concerns about transparency in the industry.

The Association of National Advertisers found that media companies are using rebates to reward ad agencies for buying a certain amount of advertising time or space on behalf of their clients, the people said. The group found that the practice was widespread within the sample it studied.

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