What Agency in the U.S. can I call to complain about an Internet service provider?

Have you complained to the ISP? Can you try it from someone else's house. Slow modem connections can be the fault of the ISP, but much more likely it is noise on the phone line, a crummy modem, or other computer problems that mess your speed. The ISP may be able to help diagnose the problem. Otherwise you best option is to change providers.