Since public revelations that the National Security Agency is collecting telephone records and reviewing Internet communications in the U.S. and abroad, officials have been making the case that the programs are vital. They argue that the tactics match the new ways terrorists are planning and communicating.
There was a time when America's enemies conspired face-to-face, or communicated through couriers, or by leaving messages for each other somewhere. But in the digital age, that has changed.
FBI Director Robert Mueller made that point back in 2008, as Congress considered whether to amend the Foreign Intelligence Surveillance Act.
"In this day and age, our ability to gain intelligence on the plans, the plots of those who wish to attack us is dependent on us obtaining information relating to cellphones, Internet, email, wire transfers, all of these areas," he said.
If all the action was in that electronic space five years ago, it's even more so today, as intelligence and security officials constantly point out.
Speaking in February, the NSA's general counsel, Rajesh De, threw out some figures on the explosive growth in communication data.
"More data crosses the Internet every second today than existed on the Internet 20 years ago. Global mobile traffic grew 70 percent last year alone," he said.
Officials say these trends highlight the challenge facing spy agencies: With so much communication now taking place in the digital world, intelligence officers have to be able to follow that communication.
James Bamford, the author of several books on the NSA, says spies used to focus on getting human sources inside an organization — agents who could report on what people in the organization were saying and doing. But human sources no longer matter so much, Bamford says. Intelligence officers use new approaches because their adversaries are interacting in new ways.
"During the day, they're on cellphones, or they're on email, or they're on social-networking sites. By intercepting that information, you develop patterns and look at who these people might be involved with," he says.
To justify the NSA's collection of telephone records and its selective monitoring of online communication overseas, U.S. officials cite these "revolutionary" changes in the information space. John Negroponte was the director of National Intelligence when wiretapping programs were expanded during the Bush administration. He defends the NSA's new emphasis.
"I'd say it's a testament to how surveillance methods have kept up with the geometric progression of these communication methods," he says.
Congressional critics of the expanded surveillance operations say they're not convinced that these programs have really proved their value in fighting terrorism. They ask whether other types of intelligence gathering might be just as effective.
Negroponte, who served as U.S. ambassador to Iraq, says no one method is sufficient. He recalls how in 2006, the combination of different intelligence sources led the U.S. military to the head of al-Qaida in Iraq, Abu Musab al-Zarqawi.
"I believe his phone number was detected through human intelligence. Somebody gave us his phone number. Then, that phone number was monitored through signals intelligence. And then his movements were tracked by geo-spatial intelligence — drones and so forth," he says. "So it's actually the integration of these different methodologies that actually give you the best results."
The expanded use of telephone and Internet surveillance is in part an adaptation to the information revolution. The NSA, the CIA and other agencies will defend these programs vigorously on that basis, despite concerns that Americans' privacy has been put at risk.
But that's not the whole story: It's also clear that the programs are popular in the spy business simply because they're convenient and efficient. They make intelligence gathering easier.