Crime investigators too often rely on bogus cell-signal analysis

Law-enforcement officials frequently use cellphone information to determine whether a suspect was near the scene of a crime. The FBI fields 32 agents dedicated to analyzing cell-site date, and the bureau has trained more than 5,000 state and local investigators in the methodology, according to Tom Jackson, writing in the Washington Post. And in 2012, federal and local law enforcement agencies made more than 1.1 million requests for the personal cellphone data, according to Senator Edward J. Markey.

Unfortunately, investigators too often draw the wrong conclusion from their analyses. Jackson cites the case of Lisa Marie Roberts, “…wrongly imprisoned for nearly 12 years after both Portland [OR] prosecutors and defense lawyers misunderstood cellphone evidence, as an example of how the methodology can be misused.” Her defense attorney pressured her to plead guilty because of the supposed inviolability of the cellphone evidence.

Jackson quotes Michael Cherry, a former Bell Labs and NASA consultant, as saying, “Complicated telephone technology is frequently oversold and under-defended in the courtroom.” And Jackson quotes Edward J. Imwinkelried, a University of California at Davis law professor and expert in the use of scientific evidence, as saying, “As well-intentioned and completely honest as some of the prosecution experts are, I don’t think they have that deep understanding of how the [phone] network systems operate.”

What's at issue is not the cellular system's ability to locate a phone in real-time using techniques like multi-tower triangulation and geolocation. The problem stems from single-tower signal strengths gathered days or weeks after an incident. Investigators assume, often wrongly, that a phone will connect to the nearest tower with the strongest signal strength and that a tower's maximum range will be two miles.

But, writes Jackson, AT&T engineer Trin Lopez testified in a 2012 California murder trial that towers in Los Angeles can reach up to 20 miles, depending on factors such as weather, time of day, type of equipment, and call traffic. He added, “It is not possible for anyone to reliably determine the particular coverage area of a cell-tower antenna after the fact based solely on historical cell-tower location data or call-detail records.”

Jackson also quotes Jeff Fischbach, a forensic expert, as saying, “There are so many different factors [involved] that two cellular devices stationed next to each other making phone calls at the same moment could still get different towers…. I’ve seen proof that two individuals, subscribed to the same cellular provider, standing next to each other—on surveillance—can still get different towers.”

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