Pressure is mounting on the Department of Justice to investigate alleged antitrust violations committed by Comcast.

The American Cable Association (ACA), a lobbying group for midsize cable operators, sent a letter on Nov. 6, demanding an investigation citing the “substantial competitive harm” it has already provided consumers.

The letter was addressed to Makan Delrahim, assistant attorney general for the Antitrust Division, and a hawk on antitrust issues.

Under Delrahim, the DOJ is appealing the AT&T-Time Warner deal over concerns the merger will inhibit competition. Both AT&T-Time Warner and Comcast-NBCU are vertical mergers, meaning the deals marry a content producer with a content distributor.