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He said agents would have to weigh “the larger social or moral good” in deciding whether “to participate in an action that otherwise you would find wrong.”

“In each individual agent’s mind, there has to be a clear understanding of how far you’ll go to penetrate an organization,” he said, suggesting one scenario in which a terrorist group was planting bombs in the mail system but U.S. intelligence officials had not identified the commanders of the conspiracy.

“With counterterrorism, you might know it’s going to happen, say another bomb in a mailbox. If you knew about it and were ‘inside,’ would you let it go off? There might be a good reason to do that,” he said.