In a declassified internal report from 2012, NSA tracked 2,776 compliance incidents — meaning NSA analysts made errors in surveillance. But 2,065 of those incidents were cases of these "roamers" — suspects who had been under surveillance who managed to enter the U.S. legally, or illegally.
"It's a foreign phone, it's pinging off foreign networks," Rogers said. "The suspect may turn it off. The suspect gets here. Now all of the sudden, the next thing they know, they (the NSA) are picking it up, but it's in Brooklyn. ... But they've been listening to it for two days. They have to turn it off, and then report it as an incident."
The NSA's mandate forbids it from spying on anyone inside the U.S., except in rare instances when the agency is allowed to spy on foreigners after making a case to a FISA court judge.
In most cases, it falls to the FBI to track such roaming suspects — and when the NSA calls to report a roamer, it has already stopped surveillance, and the clock has already started on the suspect's opportunity to disappear inside the U.S.
"Detect, cease," said one U.S. official, describing what the NSA does when it realizes its quarry is inside the States. The agency also has to throw out whatever it has collected — emails, phone calls or more — from the point it determines the suspect entered U.S. territory, the official said, speaking on condition of anonymity because the official was not authorized to speak publicly.
The FBI then has to decide quickly if the person is dangerous enough to start following electronically. If the suspect was, say, al-Qaida's top bomb maker, the agents would scramble to build a case showing "probable cause" to follow him. The FBI can directly contact the attorney general to ask for emergency authority to follow the suspect for seven days. The agents then have to present the government's case to the FISA court, for retroactive approval of the spying.