"[Snowden] was very concerned that the government would find out what it was that he was planning on doing before he got a chance to meet with us and turn over the materials," Greenwald said. "I remember at the time thinking that he was probably a little paranoid in thinking there was this massive surveillance state that would be monitoring what it was that we were doing."
"Then once I did get the documents, and was able to read through them, and report on them, I realized that actually it was a byproduct of my ignorance, not his paranoia -- that we really do live in a kind of a surveillance state and he was quite right to be that worried."
Before meeting with Snowden, Greenwald said he asked him to send some of the top-secret government documents he claimed to possess to prove what he had was real. When he received the first batch -- about two dozen documents or so -- Greenwald remembered "barely being able to breathe" because "they were so overwhelming." He then immediately got on a plane to Hong Kong to meet Snowden, who gave him thousands more.
"Once I saw the ... full first set of the archive that he provided, the thousands and thousands of top-secret documents, that's when I knew that this was the most significant leak in national security history," Greenwald said.
Days after the first reports were posted online, Snowden revealed himself as the source of the leaks in a video. He later slipped out of Hong Kong to Russia, where he was granted temporary asylum.
Greenwald believes the Internet, where huge amounts of information can be downloaded in minutes, is what made the Snowden leak possible and the evolution of social media into a sort of information weapon helped the leak spread quickly.
"[The Internet has] really made it almost impossible for the United States government to safeguard huge amounts of information that it wants to keep secret from its own population," Greenwald said. "What the Internet and social media, in particular have done, is it has enabled people like me to reach large numbers of people."
Greenwald no longer works for the Guardian and lives in Brazil. He said there is "no question" that he is under "continuous electronic surveillance," which, if true, doesn't seem to surprise him.
"That's just the nature of the U.S. surveillance state, is they try and invade anyone's communication who they think in any way is opposing what is they're doing," he said. "And that's one of the things that make it so menacing."
Snowden, living a quiet, mysterious life in Russia, is wanted in the U.S. on espionage-related charges.
ABC News' Lee Ferran contributed to this report.