Within weeks of the attacks, the giant ears of the National Security Agency, always pointed outward toward potential enemies, turned inward on the American public itself. The Foreign Intelligence Surveillance Court, established 23 years before to ensure that only suspected foreign agents and terrorists were targeted by the NSA, would be bypassed. Telecom companies, required by law to keep the computerized phone records of their customers confidential unless presented with a warrant, would secretly turn them over in bulk to the NSA without ever asking for a warrant.

Around the country, in tall, windowless telecom company buildings known as switches, NSA technicians quietly began installing beam-splitters to redirect duplicate copies of all phone calls and email messages to secret rooms behind electronic cipher locks.

There, NSA software and hardware designed for “deep packet inspection” filtered through the billions of email messages looking for key names, words, phrases and addresses. The equipment also monitored phone conversations and even what pages people view on the Web — the porn sites they visit, the books they buy on Amazon, the social networks they interact with and the text messages they send and receive.

Because the information is collected in real time, attempting to delete history caches from a computer is useless.

At the NSA, thousands of analysts who once eavesdropped on troop movements of enemy soldiers in distant countries were now listening in on the bedroom conversations of innocent Americans in nearby states.

“We were told that we were to listen to all conversations that were intercepted, to include those of Americans,” Adrienne Kinne, a former NSA “voice interceptor,” told me. She was recalled to active duty after Sept. 11.