The four major agency holding companies are now ensnared in the Department of Justice's probe into ad production practices after WPP on Monday confirmed that three of its subsidiaries have received subpoenas.

Omnicom Group on Friday said two of its subsidiaries had received DOJ subpoenas, while Publicis Groupe said one of its subsidiaries had gotten one. Interpublic Group earlier confirmed that one of its standalone domestic agencies had been contacted.

WPP confirmed its involvement on Monday morning, issuing this statement: "WPP confirms that, similarly to Interpublic, Omnicom and Publicis, three of its subsidiaries have received subpoenas from the U.S. Department of Justice Antitrust Division concerning the Division's ongoing investigation of video production and post-production practices in the advertising industry. WPP and its subsidiaries are fully cooperating with the enquiries."

The DOJ is examining ad agencies for allegedly fixing bids by encouraging production houses to increase their prices so that contracts are awarded to agencies' in-house teams. The Wall Street Journal first reported the probe in early December. The investigation is being led by Rebecca Meiklejohn, who in ad industry circles is known for guiding the Grey/Color Wheel bid-rigging scandal in the early 2000s that sent six executives to prison.