The Federal Bureau of Investigation is looking into media trading and transparency in the US advertising market.
Campaign has learned from multiple sources that representatives from the FBI have been contacting people with knowledge of the ad industry and asking for their help in recent months.
It is thought that the FBI, the law enforcement arm of the Department of Justice, is gathering information.
One source suggested the FBI is taking an interest in certain companies and the role of senior leaders in those companies.
It is not clear if the FBI has opened a formal investigation into media trading and transparency because a spokesman told Campaign that he could not comment or offer any guidance.
The FBI has a policy of not confirming or denying the existence of any investigation.
There has been speculation that the DoJ might launch an investigation after the Association of National Advertisers (ANA) published a report by private investigators K2 Intelligence about "non-transparent" practices at US media agencies in June 2016.
The report did not identify any companies by name but suggested some agency groups were receiving secret rebates from media owners and marking up the cost of ads at a profit.
Agency groups maintained that they had done nothing wrong in the wake of the ANA report.
Separately, advertisers, agencies and publishers have raised concerns about the digital media supply chain because of the rise of online ad fraud in recent years.
The FBI is the lead US government agency that investigates corporate fraud and it has significant powers to look into white-collar crime, including "illicit transactions designed to evade regulatory oversight" and "kickbacks".
Some US advertisers believe the issue of undisclosed media rebates remains a problem.
McKinsey, the management consultants, published a report last month about media rebates.
"In the face of advertisers’ push for more transparency, some agencies have continued former practices and others have figured out how to continue receiving the equivalent of rebates, but under a different name and structure," McKinsey warned.
Industry observers say media trading has become complex and can sometimes involve payments in multiple countries or offshore.
The ANA report only examined media agency practices in the US.
The DoJ has already opened an investigation into alleged bid-rigging by agencies in the US advertising production sector, which began at the end of 2016.
All of the agencies denied wrongdoing.
Rebecca Meiklejohn, the DoJ attorney who is leading the investigation into advertising production, has a long record of prosecuting figures in the ad industry for wrongdoing and winning convictions.
There has been little news about the progress of her investigation.