Dept. of JusticeThe U.S. Department of Justice has opened a probe to determine whether Comcast's ad sales practices violate federal trust law. The Wall Street Journal first reported the story.

Specifically, the Justice Department’s antitrust division has issued a document known as a “civil investigative demand” — similar to a subpoena — to look into whether Comcast attempted to monopolize the sale of cable ads in local service areas where Comcast offers service.

These zones comprise what’s known in the industry as the “spot market,” deals made between cable companies and channels to reserve a limited number of ad slots in their programming for local advertising. These slots give regional advertisers the ability to run ads side-by-side with national advertisers over national programming.

Local ads are typically sold with the aid of cooperatives called “interconnects,” ads shops run by multiple cable companies that offer service in a particular region. These cooperatives are intended to keep costs low for local advertisers and increase ad sale efficiency, allowing rival cable companies to coordinate and arrange how local ads are sold and inserted into programming in these markets. It also means larger cable companies like Comcast negotiate local ads sales on behalf of smaller cable providers. Comcast owns interconnect Spotlight, and as the largest cable operator, manages interconnects in 26 of the top 50 TV markets in the country, according to the Wall Street Journal.

The Justice Department probe is concerned with whether Comcast monopolizes local cable ad markets in regions where it offers cable service, and whether the company overstepped its boundaries in negotiating with competing cable providers the required use of its interconnect Spotlight for the sale of ads.

Comcast in February called off its much-maligned $45 billion acquisition of Time Warner Cable, which would have effectively established Comcast as a 33 million customer company. That yearlong plan was terminated after the Department of Justice announced that it planned to file an antitrust lawsuit against the two companies, given the alleged effect the merger would have on competition in the broadband and cable markets.