Comcast, the nation's second-biggest cable TV provider, poses "a much greater threat to competition" than AT&T's recent merger with Time Warner — and the Justice Department should open a formal investigation, according to a collection of small and independent cable companies.

The request to the Justice Department by Comcast's rivals adds renewed pressure on the agency to revisit Comcast's 2011 merger with NBCUniversal, a deal that critics said opened up numerous opportunities for anti-competitive misconduct.

For years, a number of government restrictions attached to the deal had limited Comcast's ability to harm consumers, the letter by the American Cable Association said. But those restrictions expired over the summer, the group said, opening the door to questionable business tactics such as withholding NBC's television programming from competing cable providers.

"Without any conditions, Comcast-NBCUniversal can act with impunity, and DOJ should expect this to happen," ACA wrote in the letter, which was dated Nov. 6 and released on Monday. "We have heard from ACA members that they fear that Comcast-NBCUniversal may restrict, if it is not already restricting, their ability to access Hulu."

The cable organization has more than 700 members, including more than 20 in Missouri; its current vice chairman is Patricia Jo Boyers of Poplar Bluff-based Boycom.

Comcast, a part owner of Hulu, on Monday called the letter an "inappropriate attempt to gain leverage in the commercial marketplace."