Former intelligence worker Edward Snowden, 29, revealed himself as the source of documents outlining a massive effort by the NSA to track cell phone calls and monitor the e-mail and Internet traffic of virtually all Americans. He says he just wanted the public to know what the government was doing. "Even if you're not doing anything wrong, you're being watched and recorded," he said. While he has not been charged, the FBI is conducting an investigation into the leaks. 
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. 
Starting in 1932, the U.S. Public Health Service studied untreated syphilis in black men who thought they were getting free health care. The patients weren't told of their affliction or sufficiently treated. Peter Buxtun, who worked for the Public Health Service, relayed information about the Tuskegee syphilis experiment to a reporter in 1972, which halted the 40-year study. His testimony at congressional hearings led to an overhaul of the Health, Education and Welfare rules concerning work with human subjects. A class-action lawsuit was settled out-of-court for $10 million, with the U.S. government promising free medical care to survivors and their families. Here, participants talk with a study coordinator. 
In 2005, retired deputy FBI director Mark Felt revealed himself to be the whistle-blower "Deep Throat" in the Watergate scandal. He anonymously assisted Washington Post reporters Carl Bernstein and Bob Woodward with many of their stories about the Nixon administration's cover-up after the June 1972 break-in at the Democratic National Committee headquarters. The stories sparked a congressional investigation that eventually led to President Nixon's resignation in 1974. The Post won a Pulitzer Prize for its coverage. Felt was convicted on unrelated conspiracy charges in 1980 and eventually pardoned by President Ronald Reagan before slipping into obscurity for the next quarter-century. He died in 2008 at age 95. 
Mordechai Vanunu, who worked as a technician at Israel's nuclear research facility, leaked information to a British newspaper and led nuclear arms analysts to conclude that Israel possessed a stockpile of nuclear weapons. Israel has neither confirmed nor denied its weapons program. An Israeli court convicted Vanunu in 1986 after Israeli intelligence agents captured him in Italy. He was sentenced to 18 years in prison. Since his release in 2004, he has been arrested on a number of occasions for violating terms of his parole. 
President Ronald Reagan addresses the media in 1987, months after the disclosure of the Iran-Contra affair. A secret operation carried out by an American military officer used proceeds from weapons sales to Iran to fund the anti-communist Contras in Nicaragua and attempted to secure the release of U.S. hostages held by Iran-backed Hezbollah in Lebanon. Mehdi Hashemi, an officer of Iran's Islamic Revolutionary Guards Corps, leaked evidence of the deal to a Lebanese newspaper in 1986. Reagan's closest aides maintain he did not fully know, and only reluctantly came to accept, the circumstances of the operation. 
Tobacco industry executive Jeffrey Wigand issued a memo to his company in 1992 about his concerns regarding tobacco additives. He was fired in March 1993 and subsequently contacted by "60 Minutes" and persuaded to tell his story on CBS. He claimed that Brown & Williamson knowingly used additives that were carcinogenic and addictive and spent millions covering it up. He also testified in a landmark case in Mississippi that resulted in a $246 billion settlement from the tobacco industry. Wigand has received public recognition for his actions and continues to crusade against Big Tobacco. He was portrayed by Russell Crowe in the 1999 film "The Insider." 
For 10 years, Frederic Whitehurst complained mostly in vain about practices at the FBI's world-renowned crime lab, where he worked. His efforts eventually led to a 1997 investigation that found lab agents produced inaccurate and scientifically flawed testimony in major cases, including the Oklahoma City and World Trade Center bombings. The Justice Department recommended major reforms but also criticized Whitehurst for "overstated and incendiary" allegations. He also faced disciplinary action for refusing to cooperate with an investigation into how some of his allegations were leaked to a magazine. After a yearlong paid suspension he left the bureau in 1998 with a settlement worth more than $1.16 million. 
FBI whistle-blower Coleen Rowley accused the bureau of hindering efforts to investigate a suspected terrorist that could have disrupted plans for the September 11, 2001, terror attacks. In 2002 she fired off a 13-page letter to FBI Director Robert Mueller and flew to Washington to hand-deliver copies to two members of the Senate Intelligence Committee and meet with committee staffers. The letter accused the bureau of deliberately undermining requests to look into Zacarias Moussaoui, the only person convicted in the United States of playing a role in the attacks. She testified in front of Congress and the 9/11 Commission about the FBI's mishandling of information. Rowley was selected as one of Time magazine's People of the Year in 2002, along with whistle-blowers Sherron Watkins of Enron and Cynthia Cooper of WorldCom. 
Sherron Watkins, a former vice president at Enron, sent an anonymous letter to founder Kenneth Lay in 2001 warning him the company had accounting irregularities. The memo eventually reached the public and she later testified before Congress about her concerns and the company's wrongdoings. More than 4,000 Enron employees lost their jobs, and many also lost their life savings, when the energy giant declared bankruptcy in 2001. Investors lost billions of dollars. An investigation in 2002 found that Enron executives reaped millions of dollars from off-the-books partnerships and violated basic rules of accounting and ethics. Many were sentenced to prison for their roles in the Enron scandal. 
Cynthia Cooper and her team of auditors uncovered massive fraud at WorldCom in 2002. They found that the long-distance telephone provider had used $3.8 billion in questionable accounting entries to inflate earnings over the past five quarters. By the end of 2003, the total fraud was estimated to be $11 billion. The company filed for bankruptcy protection and five executives ended up in prison. Cooper started her own consulting firm and told her story in the book "Extraordinary Circumstances: The Journey of a Corporate Whistleblower." 
In 2003, federal air marshal Robert MacLean anonymously tipped off an MSNBC reporter that because of budget concerns, the TSA was temporarily suspending missions that would require marshals to stay in hotels just days after they were briefed about a new "potential plot" to hijack U.S. airliners. The news caused an immediate uproar on Capitol Hill and the TSA retreated, withdrawing the scheduling cuts before they went into effect. MacLean was later investigated and fired for the unauthorized disclosure of "sensitive security information." 
Joe Darby is the whistle-blower behind the Abu Ghraib prison abuse scandal in Iraq. He says he asked Army Reserve Spc. Charles Graner Jr. for photos from their travels so he could share them with family. Instead, he was given photos of prisoner abuse. Darby eventually alerted the U.S. military command, triggering an investigation and global outrage when the scandal came to light in 2004. Graner was sentenced to 10 years in prison for his part in the abuse. He was released in 2011 after serving 6½ years of his sentence. The military and members of Darby's own family ostracized him, calling him a traitor. Eventually he and his wife had to enter protective custody. 
The New York Times reported in 2005 that in the months after the September 11, 2001, attacks, President George W. Bush authorized the U.S. National Security Agency to eavesdrop without a court warrant on people in the United States, including American citizens, suspected of communicating with al Qaeda members overseas. The Bush administration staunchly defended the controversial surveillance program. Russ Tice, an NSA insider, came forward as one of the anonymous sources used by the Times. He said he was concerned about alleged abuses and a lack of oversight. Here, President Bush participates in a conversation about the Patriot Act in Buffalo, New York, in April 2004. 
Army Pfc. Bradley Manning is accused in the largest leak of classified documents in U.S. history. His court-martial began on June 3. He has pleaded guilty to 10 of 22 charges against him and could face up to two decades in jail. He has pleaded not guilty to the most serious charge - that of aiding U.S. enemies, which carries the potential for a life sentence. At a February proceeding, Manning read a statement detailing why and how he sent classified material in 2010 to WikiLeaks, a group that facilitates the anonymous leaking of secret information. 
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- National Security Agency chief says NSA has prevented "dozens of terrorist events"
- Peter Bergen says publicly available information minimizes value of NSA surveillance
- Bergen: Most effective weapons against terror are traditional law enforcement techniques
Editor's note: Peter Bergen is CNN's national security analyst and a director at the New America Foundation. David Sterman is a graduate student at Georgetown University's National Security Studies Program.
(CNN) -- Testifying before Congress on Wednesday, Gen. Keith Alexander, director of the National Security Agency, asserted that his agency's massive acquisition of U.S. phone data and the contents of overseas Internet traffic that is provided by American tech companies has helped prevent "dozens of terrorist events."
On Thursday, Sens. Ron Wyden and Mark Udall, Democrats who both serve on the Senate Select Committee on Intelligence and have access to the nation's most sensitive secrets, released a statement contradicting this assertion. "Gen. Alexander's testimony yesterday suggested that the NSA's bulk phone records collection program helped thwart 'dozens' of terrorist attacks, but all of the plots that he mentioned appear to have been identified using other collection methods," the two senators said.
Indeed, a survey of court documents and media accounts of all the jihadist terrorist plots in the United States since 9/11 by the New America Foundation shows that traditional law enforcement methods have overwhelmingly played the most significant role in foiling terrorist attacks.
This suggests that the NSA surveillance programs are wide-ranging fishing expeditions with little to show for them.
Alexander promised during his congressional testimony that during this coming week more information would be forthcoming about how the NSA surveillance programs have prevented many attacks.
A U.S. intelligence document provided to CNN by a congressional source over the weekend asserts that the dragnet of U.S. phone data and Internet information from overseas users "has contributed to the disruption of dozens of potential terrorist plots here in the homeland and in more than 20 countries around the world."
The public record, which is quite rich when it comes to jihadist terrorism cases, suggests that the NSA surveillance yielded little of major value to prevent numerous attacks in the United States, but government officials may be able to point to a number of attacks that were averted overseas.
That may not do much to dampen down the political firestorm that has gathered around the NSA surveillance programs. After all, these have been justified because they have supposedly helped to keep Americans safe at home.
NSA director: 'Dozens' of plots thwarted 
Details on NSA-thwarted plots coming Homegrown jihadist extremists have mounted 42 plots to conduct attacks within the United States since 2001. Of those plots, nine involved an actual terrorist act that was not prevented by any type of government action, such as the failed attempt by Faisal Shahzad to blow up a car bomb in Times Square on May 1, 2010.
Of the remaining 33 plots, the public record shows that at least 29 were uncovered by traditional law enforcement methods, such as the use of informants, reliance on community tips about suspicious activity and other standard policing practices.
Informants have played a critical role in preventing more than half of the plots by homegrown jihadist extremists since the 9/11 attacks, according to New America Foundation data. For instance, a group of Muslims from the Balkans living in southern New Jersey who were virulently opposed to the Iraq War told a government informant in 2007 they were plotting to kill soldiers stationed at the nearby Fort Dix army base.
Other investigations have relied on tips to law enforcement. Saudi student Khalid Aldawsari's plot to attack a variety of targets in Texas in 2011, including President George W. Bush's home in Dallas, was foiled when a company reported his attempt to buy chemicals suitable for making explosives.
Standard police work has also stopped plots. Kevin Lamar James, a convert to Islam, formed a group dedicated to holy war while he was jailed in California's Folsom Prison during the late 1990s. James' crew planned to attack a U.S. military recruiting station in Los Angeles on the fourth anniversary of 9/11 as well as a synagogue a month later.
Members of James' group financed their activities by sticking up gas stations, and their plans only came to light during the course of a routine investigation of a gas station robbery by local police in Torrance, California, who found documents that laid out the group's plans for mayhem.
In defending the role of NSA surveillance in his congressional testimony, Alexander pointed to two plots averted by his agency's programs; the plan by Denver resident Najibullah Zazi to bomb the New York subway system in 2009 and a plot the same year by Chicago resident David Coleman Headley to attack the offices of a Danish newspaper that had printed cartoons of the Prophet Mohammed that many Muslims deemed offensive.
In the Zazi case, testimony in a Brooklyn court shows that around the eighth anniversary of 9/11, U.S. officials found an e-mail address in Pakistan, believed to be connected to al Qaeda, that was in communication with njbzaz@yahoo.com, an e-mail address in the Denver area.
Officials found that Zazi was e-mailing from njbzaz@yahoo.com to get clarification from his handlers in Pakistan about how to mix ingredients for the bombs he was building in his Denver apartment. Zazi was arrested before he could carry out his plan to attack the New York subways.
This plot could well have been detected by the NSA's PRISM program, which collects the e-mails and other records of computer users outside the United States from nine U.S. technology companies, including Yahoo. But the detection of the plot does not appear to have anything to do with the government's collection of the phone records of tens of millions of Americans.
In the case of Headley, the government hasn't clarified its claim that NSA surveillance led to his detection, but Sebastian Rotella of ProPublica, who has done the most authoritative reporting on the Headley case, says that Headley came to the attention of U.S. officials because he was in contact with al Qaeda operatives in the United Kingdom who were under surveillance by the British. In other words, Headley was not detected by NSA surveillance but because of a tip from a close ally.
Moreover, conventional law enforcement methods were available to detect Headley's ties to terrorism, but they were ignored. Headley's wife warned the FBI of his involvement with the Pakistani-based militant group Lashkar-e-Taiba years before his plot to attack the Danish newspaper.
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