It simply would not be possible to shut down areas of the Internet that terrorists use to conduct malicious activity, the head of U.S. Cyber Command told a Senate panel on Tuesday.

“In a very simplistic way, people ask why can’t we shut down that part of the Internet. … Why are we not able to infiltrate that more?” Sen. Joe Manchin, D-W.Va., asked Cyber Command leader Adm. Mike Rogers during a hearing on the agency’s budget for fiscal 2017.