WPP (WPPGY) , the world's largest public relations group by market value, said Monday that it had been subpoenaed by the U.S Department of Justice, suggesting that an antitrust issue could be spreading across the industry.

The London-based WPP, which owns Ogilvy & Mather Worldwide, Y&R, Grey and J. Walter Thompson, said three of its subsidiaries have received subpoenas from the antitrust division of the DoJ concerning an ongoing investigation regarding video production and post-production in the industry.

The announcement by WPP, also the world's largest ad agency by revenue, follows Publicis (PUBGY) and Omnicom (OMC) - Get Report , the second and third-largest companies who both made similar statements on Friday. On December 7, Interpublic (IPG) - Get Report said one of its standalone agencies was contacted by the DOJ for documents regarding video production practices.

Adweek, a weekly trade publication, quoted an industry insider who said it was common practice for agencies to either mislead production companies about the rates clients will pay for their work or asked them to artificially inflate prices as favors to the agencies to help them win businesses.

WPP shares dropped dropped 0.7% to 1,769 pence at 3:30 p.m. GMT. Publicis shares were down 0.62% at €63.83 in Paris, while Omnicom shares were up 0.27% at $86.41 in New York.