The Federal Communications Commission is requesting additional information from Comcast and NBC Universal as it reviews the cable operator's plan to acquire a controlling stake in the media company.
CNBC is part of NBC Universal.
The FCC on Monday sent letters to Comcast and NBC Universal asking questions about both companies' businesses.
Among other things, regulators are seeking details about Comcast's distribution agreements for several popular cable channels that it already owns, including regional sports networks, E! Entertainment, Versus and the Golf Channel.
The FCC also wants details about Comcast's current channel lineups and about the thinking behind Comcast's decisions on which programming to carry.
From NBC Universal, the FCC is asking for details about existing agreements with a number of cable, satellite and phone companies to carry popular NBC Universal channels, including Oxygen and local NBC and Telemundo stations.
Comcast, the nation's largest cable operator, is seeking FCC and Justice Department approval to acquire a 51 percent interest in NBC Universal from General Electric.
The deal would give it control of the NBC and Spanish-language Telemundo broadcast networks; cable channels such as CNBC, Bravo and Oxygen; the Universal Pictures movie studio and theme parks; and a stake in the online video site Hulu.
The combination has raised worries among satellite companies, rival cable operators and other subscription video providers that Comcast would use its control of NBC Universal to push up prices for must-have programming or even withhold it altogether.
The deal has also sparked fears among small, independent programmers that Comcast could drop competing channels from its cable lineup or relegate them to premium tiers with fewer subscribers.
In addition, regulators are studying the likely impact of the deal on the nascent market for online video, which has the potential to threaten Comcast's core cable business.
Public interest groups are worried that Comcast could try to stifle the market's growth by withholding programming from the Internet or forcing consumers to subscribe to cable to get access to content online.