Comcast and Time Warner Cable have been making the case for their merger nearly all year. The two companies talk up their TV programming sides a lot, but most watchers know that the merger — and the future — are really all about broadband , and that market is what Comcast is poised to control on a national scale. That potential dominance has worried not only businesses and consumer advocates , but also has apparently attracted the attention of the Justice Department as well.

Reuters reports that as the DoJ review of the planned merger ticks along, the key issue emerging is how much leverage Comcast stands poised to have over the national broadband market.

The ever-popular “sources who have attended the meetings” tell Reuters that the antitrust experts at the Justice Department are asking deeply detailed questions about all of the parts of internet connectivity and video streaming that the public never really sees.

Those carriage contracts that media companies don’t want to show the FCC in public? Those negotiations about connection fees and programming deals are exactly what the DoJ is asking to see.

Reports from the meetings also say that DoJ representatives have asked specifically whether a bigger Comcast would have the incentive to “slow or meddle with” online video from other companies whose’ subscribers get their internet access from Comcast. The sources also tell Reuters that investigators have asked about content companies’ future expansion plans, as well as concerns they have over interconnection and data caps.

The CEO of Cogent, which has been part of Netflix’s very public interconnection disputes with big ISPs this year, said that the DoJ investigators are getting very into the details of the issues.

“The majority of the inquiries are around very technical data showing congestion, the timing, showing the impacts on our customers,” he told Reuters. “They’re very in the weeds.”

Consumer advocates have focused heavily on the potential harms Comcast could cause with nationwide broadband dominance when opposing the merger, both to the Justice Department and to the FCC.

A Comcast representative told Reuters that so far, total, Comcast has sent over one million pages of documentation in support of their merger to all of the agencies — presumably including municipal, state, and federal — reviewing the merger. They will probably have to submit thousands more. But even ten million pages of documentation couldn’t make this merger a good idea.

DEAL TALK: U.S. DOJ digs into Comcast’s Internet reach in merger review [Reuters]