The Federal Bureau of Investigation (FBI) has contacted the Association of National Advertisers (ANA), requesting cooperation from the trade group and its members in a criminal investigation into US media buying practices.

The ANA detailed the FBI’s probe in a statement to members and validated recent unsourced reports from The Wall Street Journal and The Guardian that the agency had opened a probe on nontransparent advertising practices , including agencies unscrupulously receiving rebates from media outlets.

In a letter to ANA members shared with AdExchanger, ANA CEO Bob Liodice said all members’ cooperation with the FBI is “entirely voluntary.”

“The starting point is to identify those advertisers which believe they have been defrauded,” Liodice wrote. “We suggest to members who think they may have been defrauded to retain counsel (through their own engagements and at their own cost), review their media buying history and contracts, perform audits for indications of fraud and get advice on their options.”

At the root of the FBI’s investigation is the ANA-commissioned 2016 K2 Intelligence Report , which detailed numerous instances of nontransparent business practices by agencies. As AdExchanger previously reported , 59 of the 117 media buying sources interviewed said they had direct experience with nontransparent practices. Thirty-four of that 59 said they had direct experience with rebates.

The report concluded such practices, including reselling media as principal at a mark-up and pocketing rebates from vendors without passing them along to clients, were “pervasive” in the United States. Many of these activities took place at the agency trading desk, where clients often opted-in to nontransparent models without fully understanding the terms of their contracts.

The ANA, which represents all major brands including Proctor & Gamble, General Motors and others, will not be coordinating interviews between its members and FBI agents. It will not serve as a conduit between federal prosecutors and its members, Liodice said. ANA members are not obligated to cooperate with the FBI and have the option to “do nothing.”

Legal firm Reed Smith, LLP is serving as the ANA’s counsel.

“We understand this to be a long-term federal investigation and from my experience they are likely investigating potential violations of mail fraud and wire frauds statutes, and possibly racketeering,” Steven A. Miller, a former prosecutor who now specializes in white collar crime at Reed Smith, told AdExchanger. “No one’s been accused of a crime, and it’s at the investigation stage.”

It’s unclear whether clients will want to cooperate with the FBI investigation or handle matters quietly themselves. Many marketers have been reworking their contracts with agencies over the past two years to take ownership of their technology and data platforms and provide greater transparency into agency spend.

Many clients have cited transparency as a reason to take programmatic buying responsibilities in-house, according to a study by the IAB. According to the ANA just 40% of clients are comfortable with the transparency they receive from programmatic media investments.

Alison Weissbrot contributed