Interviews with two NSA officials by National Public Radio reveal a tragic irony of Edward Snowden's theft and leaking of classified documents: He was just doing his job.
One of the lessons learned from the investigations of the 9/11 Commission was that there was insufficient sharing of intelligence information. In order to facilitate such sharing, the NSA created a file sharing area on the Agency's intranet site. NSA officials told NPR that all of the documents Snowden has leaked came from that sharing area.
"Unfortunately for us," one official said, "if you had a top secret SCI [sensitive compartmented information] clearance, you got access to that."
Not only did Snowden have such access, but as a System Administrator it was part of his job to search through the shared area for documents which belonged in more restrictive areas and move them. From the NPR article:
"It's kind of brilliant, if you're him," an official said. "His job was to do what he did. He wasn't a ghost. He wasn't that clever. He did his job. He was observed [moving documents], but it was his job."
It's a little more complicated than Snowden literally doing his job, although the job was the perfect cover for his activities. The article also notes that the NSA was, at the time, allowing users access to USB ports on computers which had access to the sensitive data. They have, more recently, closed off access to those ports, which Snowden likely used to copy data from NSA systems to a USB thumb drive.
Incredibly, this is the same method used by Bradley Manning long ago, but the NSA didn't react to that news. Restricting access to USB ports has been a standard feature of Windows system management for many years and there are 3rd party products that do this as well.
The NSA has implemented other practices, including document tagging, to prevent what Snowden did. The tags will restrict access to the documents and track their usage.
It's all another example of fighting the last war. The NSA, in this way, is following the example of the TSA.