Federal prosecutors in Manhattan have opened an investigation into media-buying practices in the advertising industry and have begun issuing subpoenas as part of the probe, according to people familiar with the matter.

The investigation is looking at, among other things, nontransparent ad-buying practices, including agencies receiving rebates from media outlets, the people said.

One advertising company under scrutiny in the investigation is Havas SA, the ad company owned by media conglomerate Vivendi SA, the people said.

A spokeswoman for Havas declined to comment.

Over the past several months, the Federal Bureau of Investigation has been interviewing people in the ad business about ad-buying practices and about a 2016 investigation of the industry commissioned by the Association of National Advertisers, according to people familiar with the matter.