How cellphones help catch criminals

Cellphone expert and electric engineer, Ben Levitan, gives his take on how cellphones can help authorities find perps

Fingerprints were the limit of technology for solving crimes between 1892 and the 1980s. In 1986 criminal investigators started using powerful tools to make use of the information available through DNA found at a crime scene. And now the best new tool in the investigator’s kit is cell phone evidence. It too has become a major asset in crime scene investigations.

Take the mugging an elderly woman in Mount Lebanon, Pennsylvania. Police said an 84-year-old woman was walking along Washington Road when someone snatched her purse. Crime scene investigators found no fingerprints or DNA, but they found a phone that had been dropped by the mugger. A few hours later, police arrested Nicholas Greenly on the charge of snatching the woman’s purse. Police had examined Greenly’s phone, found at the scene, and found an outgoing text message saying, “I’m ready to grab some old lady’s purse.”

Although, in this case, we can credit a dumb criminal rather than smart cell phone expert, it still demonstrates the power of cell phone evidence that we’ll explore in the blog posts to come. We’ll explore cases in the news learning what we can glean from cell phone evidence and what we cannot assume as fact. Let’s start there. At the most basic level, what is cell phone evidence and what does it tell us?

Phone companies, in the normal course of business, keep records of every phone call, text message or data use a phone makes. Phone calls are their product, and they want to track how well their system works and, of course, to bill for their product.

So every time you make or receive a call (or text), the phone company has a record of it. That record includes your phone number, the date and time of the call, and, for cell phone calls, the name of the cell tower you used. Since your cell phone always uses the closest cell tower, these records can tell investigators your GENERAL location. Remember, each cell tower is intended to cover a large area. The records cannot pinpoint your location, but through some analysis, an expert can generally put your phone within an area of about four square miles (about the size of 2,000 US football fields) at the time of the call. Cell phone evidence is clearly not DNA, but nonetheless can be very useful when properly used.

Had Nicholas Greenly not been so helpful in leaving his phone at the scene of the crime, how could we have used cell phone evidence in his case? First, the area of mugging is served by one cell tower. The cell phone company can tell police the names of all the people who used their phone in that area around the time of the crime. This may be as few as ten people. This will yield suspects and potential witnesses. Good old police work would then narrow the field. Police would quickly find out most of the people who were using their phones at that time in that area had good alibis. They were at work, at home or with friends. A few phones would remain, including Mr. Greenly’s.

Mr. Greenly’s phone records showed that his phone was in the vicinity when someone snatched the woman’s purse, but that’s the limit that the cell phone evidence could establish. With a search warrant, police could retrieve text messages from his phone and, as in this case, find out more. Even without any other clues, this would be enough to put Mr. Greenly squarely in the sights of the investigation.

More to come.