But Mr. Rusbridger told Parliament that the newspaper “went to more precautions over this material than any other story we have ever handled.”

Senior Guardian editors were initially skeptical this year when asked to hand over their cellphones before discussing Mr. Snowden’s documents, said a person with knowledge of the reporting process, who did not want to be named discussing confidential security procedures.

That soon changed when they reviewed the information Mr. Snowden had supplied, this person said. The documents, they came to realize, would be of intense interest not only to the American and British governments, from which they were taken, but also to other governments like China and Russia seeking an espionage edge and hackers seeking to embarrass either government agencies or the publications reporting on the material.

Eventually the same editors insisted that meetings be held in rooms without windows and that any electronic devices nearby be unplugged. Computers that contained the information could never be connected to the Internet. And reporters who needed to consult with colleagues in other countries about the documents had to fly them over physically and meet in person, despite the extra costs. On one occasion, Mr. Rusbridger said, encrypted documents were sent via FedEx.

Nicholas Weaver, a computer security researcher at the University of California, Berkeley, said effective countermeasures for all reporters covering such issues begin with first contact with a source.

Devices “leave fingerprints everywhere you go,” he said. “Leave all your high-tech gadgets at home; meet in a public location that’s kind of noisy, and wear a hat so you don’t get caught on camera.”

“You have to walk there, because we have this network of license plate readers now,” he said, or buy a transit ticket with cash and dispose of it afterward. As for making first contact with a sensitive source, Mr. Weaver said, “You have to wait for them to contact you.”