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. The 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. Authorities have not disclosed details on the specific government or criminal groups tied to the network at this point. In this case, he said, attackers would have been able to force that kind of shutdown at a time of their choosing.