I'm sure there were spies and secret satellites involved. (Sure were; see below.) But it was
"a media visit to Iran that helped the intelligence community reconsider its assessment" of Tehran's nuclear weapons program, intelligence officials tell USA Today's Richard Willing.
In recent years, the government has been pushing more and more resources into "open source intelligence" -- knowledge that's out there in the public sphere. Stuff out in the public sphere, in other words. "Perhaps our best source of information is the television," Rear Adm. Ronald Henderson, deputy director of operations for the Joint Staff, noted last year.
Juan Cole, on the other hand, points to a different source for the Iran info: a man named Ali Reza Asghari. (Spook86 over at In From the Cold agrees with Cole -- maybe for the first time on any subject.)
Since we're on the subject: I've read a whole lot of analysis of the NIE report since it broke. So far, the best -- and most nuanced -- one I've come across in Fred Kaplan's, in Slate.
UPDATE: "Senior officials said the latest conclusions grew out of a stream of information, beginning with a set of Iranian drawings obtained in 2004
and ending with the intercepted calls between Iranian military commanders," according to the Washington Post (via ACW).
UPDATE 2: "The only reason for a complete reversal of judgment about a nuclear program... is credible new intelligence from one or more reliable sources," according to John McCreary, a former analyst for the Joint Chiefs. **
ALSO:
* Intel Report: Iran Halted Nuke Arms in 2003
* Iran Nuke in "18 Months"? Unlikely.
* Iran's "Industrial" Nukes: Yawn
* Iran's Nukes: Time to Freak?
* Glimmers of Hope in Iran Report
* Iran's Nuclear Scientist Game