Investigators warn the system could have blacked out cellular service in a city that relies on it not only for daily life but for emergency response and counterterrorism. A broader investigation led to this discoveryThe network was uncovered as part of a broader Secret Service investigation into telecommunications threats targeting senior government officials, according to investigators. “It can’t be understated what this system is capable of doing,” said Matt McCool, the special agent in charge of the Secret Service’s New York field office. An extensive, expensive operationWhen agents entered the sites, they found rows of servers and shelves stacked with SIM cards. In this case, he said, attackers would have been able to force that kind of shutdown at a time of their choosing.