An influential Madison Avenue trade organization unveiled an investigation that alleges kickbacks, unauthorized rebates and the undisclosed resale of ad inventory at higher prices could be “pervasive” in the arcane but important world of media buying.

The Association of National Advertisers on Tuesday released the findings of a seven-month probe into what it calls “non-transparent business practices” among media agencies: Forty-one individuals interviewed by investigations firm K2 reported that they had firsthand knowledge of rebates being passed back to agencies. Of those 41 sources, 34 said the rebates were not disclosed to advertisers, were not passed through to advertisers, and may also have been demanded by agencies. The firm interviewed 150 different sources overall.

Read the full Variety article here.