US Department of Justice investigators have questioned Comcast, Time Warner Cable, and others as part of a "wide-ranging antitrust investigation into whether cable companies are acting improperly to quash nascent competition from online video," the Wall Street Journal reported today.

Justice Department officials also spoke to streaming video providers including Netflix and Hulu, as they look into whether cable company policies such as data caps prevent a level playing field between cable companies and online video providers. Since cable companies sell both Internet access and cable television service, they could place limits on data usage that impact video competitors but not their own video services. There is concern such limits could prevent consumers from switching from cable TV to Internet video.

Comcast recently raised its data caps from 250GB to 300GB, but has met controversy because its own on-demand video service for the Xbox 360 does not count against data caps, while similar services from Netflix do. The new DOJ probe is focusing on data caps from the likes of Comcast and AT&T, and in particular "whether Comcast's Xbox policy violated legal commitments made by the company in 2011 to secure antitrust approval for its takeover of NBCUniversal," the Wall Street Journal reported, citing "people familiar with the matter." The DOJ is also investigating contracts programmers sign to be distributed on cable systems, which include "most-favored nation clauses" that may favor cable companies over online video distributors.

The companies involved in the probe and the DOJ declined to comment on the investigation, the WSJ said. Advocacy groups applauded the DOJ probe, however.

“Media and telecommunications giants, which can be one and the same, should not be able to take advantage of their size and reach to eliminate competition and to harm consumers through data caps which favor some content over other based on business relationships, through contract terms that could restrict where programming can be shown, or other means," Harold Feld, legal director of Public Knowledge, said in a written statement.

The Free Press advocacy group similarly welcomed the investigation, saying "Cable operators can’t justify data caps, no matter what shifting excuses they offer. Comcast's recent floundering proves that these caps have nothing to do with congestion... Caps can discriminate against competing online video services, and even without discriminatory treatment they generally discourage customers' use of services delivered over the Internet."

A cable industry trade group, the National Cable & Telecommunications Association, also weighed in after this story was published. The NCTA sent us a statement from spokesperson Brien Dietz, reading: “Consumers are the beneficiaries of tremendous choice, competition and innovation in the video marketplace with dozens of alternatives now available for viewing content on multiple devices through a variety of service options. The innovative offerings by cable companies are positive developments for consumers and represent accepted and legitimate business practices as well as sound network management. All the industry's actions are intended solely to ensure consumers get the highest value for their subscription."

For further reading, Ars recently deconstructed Comcast's shaky claim that its own services shouldn't count against data caps because they run over a "private IP network" rather than the public Internet.