U.S. officials claim that the government’s massive data collection has protected the country from terrorist attacks. After The Guardian’s first revelations about the National Security Agency’s digital surveillance programs, Sen. Dianne Feinstein, chairman of the Senate Intelligence Committee, and Rep. Mike Rogers, head of the House Intelligence Committee, jumped to the NSA’s defense by pointing to two terrorist plots supposedly foiled by the organization’s digital surveillance programs. Lawyers and policemen involved in these cases disputed these claims, but this did not keep NSA chief Keith Alexander from taking it up a notch by raising the number of foiled attacks to more than 50, and later to 54.

These numbers are crucial for an informed debate about the digital surveillance programs. If the NSA’s digital surveillance indeed prevented 54 terrorist attacks, the public can decide whether these 54 attacks are worth their privacy. This number would suggest that the NSA’s programs are actually keeping the United States and Europe safe from terrorism.

It is far from certain, however, that the NSA is getting its numbers right.

Who Stops Terrorism?

Contrary to what one would expect given the secretive nature of intelligence operations, we actually know quite a bit about how terrorist plots in the United States and Europe are foiled. Several attacks, for instance, were discovered after law enforcement agencies picked up on suspicious (non-digital) behaviors of the plotters. Samir Azzouz, the most prolific jihadist terrorist in the Netherlands, attracted the attention of the Dutch secret service when he tried to travel to Chechnya to join the jihad against the Russians.

Other plotters gave themselves away by associating with known terrorists. For instance, a 2009 plot to attack the New York Stock Exchange came to light after one of the perpetratorscontacted a Yemeni extremist who was under FBI surveillance. The plans of Mohamed Osman Mohamud, who was arrested just before he could execute his attack against a Christmas tree-lighting ceremony in Portland, were detected in a similar manner. The FBI started following Mohamud after he e-mailed a known terrorist recruiter. Since the FBI does not have mass digital surveillance capabilities, the person Mohamud contacted was likely already under surveillance.

Najibullah Zazi’s plans for an attack against the New York subway were thwarted this way, too. British intelligence informed their U.S. counterparts that Zazi had had e-mail contact with a Pakistani radical who was being watched for involvement in a British terror plot. A fourth example involves Abdullah Ahmed Ali, the ringleader of the cell that prepared the liquid bomb attacks against transatlantic flights in 2006. He first came to the attention of MI5 after he was seen interacting with known radicals.

In other cases, the police uncovered terrorist activities after having arrested the perpetrators for unrelated crimes. A cell in London, for instance, attracted the attention of the police after its involvement in skirmishes with right-wing extremist youths. A more bizarre example concerns Ahmed Ferhani, who, apparently deeply enraged after an arrest for petty crime, told the police about his ambition to join the jihad. Several months later, he was arrested for planning an attack against a New York synagogue.

Sheer luck sometimes plays a role as well. UK police disrupted a terrorist attack against a rally of the English Defense League, a right-wing extremist organization, after pulling over the perpetrators because of a problem with their car insurance. Sometimes it’s not even the police that uncover terrorist plots. In the cases of planned attacks against the Fort Dix Army base in 2009 and against a shopping mall in Bristol in 2008, alert members of the public tipped off the police.

What about the NSA?

Admittedly we do not know how all terrorist plots have been detected. But going by what we do know, the conclusion is simple: terrorist plots have been foiled in all sorts of ways, few of which had anything to do with mass digital surveillance. True, in the case of the dismantlement of the Sauerland Cell in Germany in 2007, NSA information played a role. But whether the authorities got this information from “digital dragnet surveillance” or from more individualized and targeted monitoring is hard to tell.

It might be tempting to give the NSA the benefit of the doubt, given that the organization speaks on the basis of information that we do not have. But such dubious claims about the effectiveness of the digital surveillance programs fit seamlessly into a pattern of misinformation and deceit. The U.S. government acknowledged the existence of PRISM only after Edward Snowden had leaked details about it to The Guardian. Moreover, when the news broke, President Obama and Director of National Intelligence James Clapper tried to downplay the scale of the digital data gathering, even though we know now that the NSA is essentially making aback-up of pretty much all conceivable forms of online communication. President Obama further promised that “nobody is listening to your phone calls,” but it later became clear that the NSA can access the content of phone calls and e-mails if it so desires. Congressional oversight is poor, privacy rules are frequently broken, and the NSA liberally shares data with other intelligence agencies and foreign governments.

Against this background of disputed or outright false government claims, the public is wise to be skeptical of the NSA’s claims about the effectiveness of the digital surveillance programs. The recent revelations may be mind-boggling in their technological, legal, and procedural complexities, but the bottom line is quite simple: The first credible piece of evidence that these programs are doing any good in the fight against terrorism has yet to surface. Until such evidence is provided, the Obama administration is only eroding the trust of the citizens it is claiming to protect.