Ex-official: We lost track of terrorists

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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."

John Walker ran a father and son spy ring, passing classified material to the Soviet Union from 1967 to 1985. Walker was a Navy communication specialist with financial difficulties when he walked into the Soviet Embassy and sold a piece of cyphering equipment. Navy and Defense officials said that Walker enabled the Soviet Union to unscramble military communications and pinpoint the location of U.S. submarines at all times. As part of his plea deal, prosecutors promised leniency for Walker's son Michael Walker, a former Navy seaman. Click through the gallery to see other high-profile leak scandals the United States has seen over the years.

Military analyst Daniel Ellsberg leaked the 7,000-page Pentagon Papers in 1971. The top-secret documents revealed that senior American leaders, including three presidents, knew the Vietnam War was an unwinnable, tragic quagmire. Further, they showed that the government had lied to Congress and the public about the progress of the war. Ellsberg surrendered to authorities and was charged as a spy. During his trial, the court learned that President Richard Nixon's administration had embarked on a campaign to discredit Ellsberg, illegally wiretapping him and breaking into his psychiatrist's office. All charges against him were dropped. Since then he has lived a relatively quiet life as a respected author and lecturer.

Jonathan Pollard is a divisive figure in U.S.-Israeli relations. The former U.S. Navy intelligence analyst was caught spying for Israel in 1985 and was sentenced in 1987 to life imprisonment. The United States and Israel are discussing his possible release as part of efforts to save fragile Middle East peace negotiations, according to sources familiar with the talks.

Wen Ho Lee was a scientist at the Los Alamos Laboratory in New Mexico who was charged with 59 counts of downloading classified information onto computer tapes and passing it to China. Lee eventually agreed to plead guilty to a count of mishandling classified information after prosecutors deemed their case to be too weak. He was released after nine months in solitary confinement. Lee later received a $1.6 million in separate settlements with the government and five news agencies after he sued them, accusing the government of leaking damaging information about him to the media.

Members of the Bush administration were accused retaliating against Valerie Plame, pictured, by blowing her cover in 2003 as a U.S. intelligence operative, after her husband, former Ambassador Joe Wilson, wrote a series of New York Times op-eds questioning the basis of certain facts the administration used to make the argument to go to war in Iraq.

In 2007, Lewis "Scooter" Libby, Vice President Dick Cheney's former chief of staff, was convicted on charges related to the leak of the identity of CIA operative Valerie Plame. Libby was convicted of obstruction of justice and perjury in connection with the case. His 30-month sentence was commuted by President George W. Bush. Cheney told a special prosecutor in 2004 that he had no idea who leaked the information.

Aldrich Ames, a 31-year CIA employee, pleaded guilty to espionage charges in 1994 and was sentenced to life in prison. Ames was a CIA case worker who specialized in Soviet intelligence services and had been passing classified information to the KGB since 1985. U.S. intelligence officials believe that information passed along by Ames led to the arrest and execution of Russian officials they had recruited to spy for them.

Robert Hanssen pleaded guilty to espionage charges in 2001 in return for the government not seeking the death penalty. Hanssen began spying for the Soviet Union in 1979, three years after going to work for the FBI and prosecutors said he collected $1.4 million for the information he turned over to the Cold War enemy. In 1981, Hanssen's wife caught him with classified documents and convinced him to stop spying, but he started passing secrets to the Soviets again four years later. In 1991, he broke off relations with the KGB, but resumed his espionage career in 1999, this time with the Russian Intelligence Service. He was arrested after making a drop in a Virginia park in 2001.

Army Pvt. Bradley Manning was convicted July 30 of stealing and disseminating 750,000 pages of classified documents and videos to WikiLeaks, and the counts against him included violations of the Espionage Act. He was found guilty of 20 of the 22 charges but acquitted of the most serious charge -- aiding the enemy. Manning was sentenced to 35 years in military prison in 2013.

Former intelligence contractor Edward Snowden revealed himself as the leaker of details of U.S. government surveillance programs run by the U.S. National Security Agency to track cell phone calls and monitor the e-mail and Internet traffic of virtually all Americans. Snowden has been granted temporary asylum in Russia after initially fleeing to Hong Kong. He has been charged with three felony counts, including violations of the U.S. Espionage Act, over the leaks.

Sharing secrets: U.S. intelligence leaks

Sharing secrets: U.S. intelligence leaks

Sharing secrets: U.S. intelligence leaks

Sharing secrets: U.S. intelligence leaks

Sharing secrets: U.S. intelligence leaks

Sharing secrets: U.S. intelligence leaks

Sharing secrets: U.S. intelligence leaks

Sharing secrets: U.S. intelligence leaks

Sharing secrets: U.S. intelligence leaks

Sharing secrets: U.S. intelligence leaks

Sharing secrets: U.S. intelligence leaks

Snowden, whose massive leaks on intelligence-gathering programs resulted in espionage charges from the U.S. government, has been living in Russia for the past year. He said during an interview in May that he had no choice other than leaking the highly classified information since he believed the Constitution was being violated.

Government officials, from President Barack Obama down, have uniformly decried the disclosures, though in the years since the information became public there's been open debate about the mass surveillance programs that came to light.

New terror threats have also emerged since Snowden's disclosures, including the ISIS terror group operating in Syria and Iraq, and the al Qaeda offshoot Khorasan that's based in Syria.

Olsen acknowledged terrorists may have suspected their communications were being intercepted before the leaks. But he said Snowden's information confirmed the practice and made it easier for groups to find ways around U.S. surveillance.

Olsen: 'Imminent threat' remains