Brian Roberts, chairman and chief executive officer of Comcast Corp., attends the Allen & Company Sun Valley Conference on July 9, 2014 in Sun Valley, Idaho. Scott Olson/Getty Images The US government is launching an investigation into whether Comcast unfairly monopolizes the cable advertising sales market, the Wall Street Journal reports.

According to a civil investigative demand seen by The Wall Street Journal, the US Justice Department is looking at whether Comcast is holding back competition in the "spot" cable ad sales market, which could be violating federal antitrust law.

The spot market represents the short amount of broadcasting time in which cable TV channels allow pay-TV operators — the likes of Comcast — to sell local TV advertising, targeted to specific geographies or networks during the commercial breaks.

Because the audience for spot advertising is smaller, spot ad rates tend to be a lot lower than buying national TV commercials. Spot advertising only makes up a small proportion of the total $71.1 billion US TV ad market.

The Wall Street Journal says the investigation focuses on two areas: "monopolization or attempted monopolization" and whether Comcast's deals with competitors in the pay-TV market are "an unlawful restraint on trade."

The Wall Street Journal explains what the investigation will cover:

The probe isn’t about the size of Comcast’s footprint for providing cable TV service. The question has more to do with whether Comcast is wielding outsize influence when it comes to selling local TV advertising on cable, in two main ways: Comcast takes the lead on negotiating with advertisers on behalf of rival pay-TV providers in many markets. Comcast also owns a majority stake in one of the main companies that helps national advertisers buy commercial time from cable providers in local markets.

Business Insider has contacted both Comcast and the DoJ for comment. The DoJ declined to comment.

Comcast provided Business Insider with this statement:

The American advertising market is robustly competitive, and advertisers have never had more choices to reach consumers. Local cable advertising competes with local broadcast TV, radio, newspapers, outdoor display, and the rapidly growing digital marketplace. All multichannel-video providers – or MVPDs – account for only about 7% of local advertising sales.

To better compete with local advertising platforms with significant scale, like a broadcaster or web app that can sell an entire geographic market, and to provide more and better choices to advertisers, MVPDs have long worked together through local interconnect arrangements to sell local advertising. These interconnects increase efficiency and help keep costs down for advertisers and are responsive to the needs of major local advertisers. We believe these long-standing industry practices are good for advertisers and consumers, and we and other MVPDs are continuing to provide these important services to our clients.

We plan to cooperate fully with the Department of Justice’s inquiry.