“We’ve lost collection against some individuals, people that we were concerned about we are no longer collecting their communications,” Matt Olsen, who until July led the National Counterterrorism Center, told CNN’s Jim Sciutto. “We lost insight into what they were doing.”

Olsen said the revelations made public by former NSA contractor Edward Snowden had changed the way terrorists communicate, causing them to fall out of the U.S. government’s sight.

“They’ve changed how they encrypt their communications and adopted more stringent encryption techniques,” he said. “They’ve changed service providers and email addresses and they’ve, in some cases, just dropped off all together.”