The officials said they use material from multiple sources — allies, agents, informants and other investigations — to provide rolling targeting information for the PRISM program. And, they said, if the Yahoo address had not been included, Zazi might not have been identified just days before the attacks were set to occur.
In testimony before Congress on Tuesday, senior intelligence and law enforcement officials said that recently revealed surveillance programs have disrupted more than 50 “potential terrorist events,” including at least 10 plots with a connection in the United States.
The Zazi case was one of four that officials used in recent days to defend the effectiveness of the surveillance programs. One of the others was a planned attack on a Danish newspaper that involved a Pakistani American, David Headley.
Sean Joyce, the deputy director of the FBI, described the other two potential attacks Tuesday in testimony before the House Intelligence Committee.
In one, Joyce said, the NSA was monitoring “a known extremist in Yemen” when it learned that the individual was in contact with a man in Kansas City, Mo. Joyce said that Khalid Ouazzani and two co-conspirators were engaged in a nascent plot to bomb the New York Stock Exchange. Ouazzani pleaded guilty in 2010 to supporting a terrorist organization, bank fraud and overseas money laundering. His co-conspirators also pleaded guilty to terrorism charges.
In the other incident, telephone-call records helped identify a San Diego man who was financing a terrorist group overseas, apparently al-Shabab in Somalia. Joyce did not specifically identify the case, but it appears to have led to the successful prosecution of four Somali immigrants this year who were planning to help finance a terrorist organization in Somalia.
“Investigating terrorism is not an exact science; it’s like a mosaic,” Joyce said. “And we try to take these disparate pieces and bring them together to form a picture. There are many different pieces of intelligence. We have assets. We have physical surveillance. We have electronic surveillance through a legal process, phone records through additional legal process, financial records. Also, these programs that we’re talking about here today, they’re all valuable pieces to bring that mosaic together.”
Officials acknowledged that intelligence collected from U.S. phone records under a program authorized by Section 215 of the USA Patriot Act is less compelling and that the case for that extensive surveillance is harder to make.
The NSA’s ability to intercept “the contents of e-mail communications of bad guys overseas provides a more lucrative set of information” about terrorist activity than its access to phone records of millions of Americans, one U.S. official said.
Ellen Nakashima and Julie Tate contributed to this report.