The only problem is it doesn't work that way, as the case of Brandon Mayfield demonstrates:
But there’s another danger that Snowden didn’t mention that’s inherent in the government’s having easy access to the voluminous data we produce every day: It can imply guilt where there is none. When investigators have mountains of data on a particular target, it’s easy to see only the data points that confirm their theories — especially in counterterrorism investigations when the stakes are so high — while ignoring or downplaying the rest. There doesn’t have to be any particular malice on the part of investigators or analysts, although prejudice no doubt comes into play, just circumstantial evidence and the dangerous belief in their intuition. Social scientists refer to this phenomenon as confirmation bias, and when people are confronted with data overload, it’s much easier to weave the data into a narrative that substantiates what they already believe. Criminologist D. Kim Rossmo, a retired detective inspector of the Vancouver Police Department, was so concerned about confirmation bias and the investigative failures it causes that he warned police officers in Police Chief magazine to always be on guard against it. “The components of confirmation bias,” he wrote, “include failure to seek evidence that would disprove the theory, not utilizing such evidence if found, refusing to consider alternative hypotheses and not evaluating evidence diagnosticity.”
Despite finding that Mayfield’s print was not an identical match to the print left on the bag of detonators, FBI fingerprint examiners rationalized away the differences, according to a report by the Department of Justice’s Office of the Inspector General (OIG). Under the one discrepancy rule, the FBI lab should have concluded Mayfield did not leave the print found in Madrid — a conclusion the SNP reached and repeatedly communicated to the FBI. The FBI’s Portland field office, however, used that fingerprint match to begin digging into Mayfield’s background. Certain details of the attorney’s life convinced the agents that they had their man. Mayfield had converted to Islam after meeting his wife, an Egyptian. He had represented one of the Portland Seven, a group of men who tried to travel to Afghanistan to fight for al Qaeda and the Taliban against U.S. and coalition forces in a child custody case. He also worshipped at the same mosque as the militants. In the aftermath of 9/11, these innocent associations and relationships, however tangential, were transformed by investigators into evidence that Mayfield wasn’t a civic-minded American, but a bloodthirsty terrorist intent on destroying the West.
FBI agents broke into Mayfield’s home and law office. They rifled through documents protected by attorney-client privilege, wiretapped his phones, analyzed his financial records and web browsing history, and went through his garbage. They followed him wherever he went. Despite all this, the FBI never found a smoking gun connecting him to Madrid. They did, however, find Internet searches of flights to Spain and learned that he once took flying lessons. To FBI agents already convinced of his guilt, this was all evidence of Mayfield’s terrorist heart. The Web searches, however, were mundane. His daughter had to plan a fictional vacation for a school project. Flight lessons were indicative of nothing more than Mayfield’s interest in flying.
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