"The intent of the programs are to find terrorist actors and report that to the FBI," he said.
One audience member briefly heckled Alexander and characterized his descriptions of the spy programs as "bullshit." For the most part, though, the capacity crowd appeared to appreciate Alexander's efforts to explain the program.
At one point the crowd broke into broad applause when he asked the heckler to read up the Constitution after being urged to do the same by the heckler.
He also described as "bullshit" suggestions that the NSA might be exceeding legal limits in its use of phone metadata and data collected under the PRISM program. Congressional investigations have found that the program is operating legally, he said.
Concerns about audience reaction to an NSA presence had earlier prompted organizers of the DEFCON hacker conference to disinvite government employees.
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