Ex-counterterror chief: U.S. lost track of terrorists …

By Kevin Liptak, CNN

updated 9:39 AM EDT, Wed October 22, 2014

Watch CNN's full interview with Matt Olsen during "The Situation Room," airing from 5-7 p.m. ET.

Washington (CNN) -- Ripping the cover off top-secret U.S. surveillance programs pushed foreign terrorists underground and out of intelligence services' reach, a former top intelligence official said Tuesday.

"We've lost collection against some individuals, people that we were concerned about we are no longer collecting their communications," Matt Olsen, who until September 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."

Sharing secrets: U.S. intelligence leaks

Sharing secrets: U.S. intelligence leaks

Sharing secrets: U.S. intelligence leaks

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Ex-counterterror chief: U.S. lost track of terrorists ...