Can cell tower data be trusted as evidence?

Since GIJC 2015 is coming up, I thought i’d share the project i’ve been working on for the last six months. Maybe somebody can be inspired.

I wanted to research if cell tower data be trusted as evidence in criminal cases. Cell tower data is information about what cell tower (‘telefonmast’ or ‘mobilmast’ in danish) a cell phone connected through during a call, text message or data session.

During a conversation with a lawyer a few years ago, she mentioned that cell tower data isn’t precise, even though it’s used in many criminal cases (a little more than 4.000 last year in Denmark). At the time I didn’t think about, but after a while it dawned on me, that it was an interesting subject. So I started digging. When I had done the first pilot test in the project, the Serial podcast aired debating the same subject. I ended up writing my BA thesis about the subject at the Danish School of Media and Journalism (DMJX). It got the highest grade (12) and was awarded with the E.N. Ritzaus Mindelegat-award. The danish daily Information bought the articles and some follow-ups, which I’ll describe here.

The main case is Mette Hansen, a danish veterinarian, who was falsely accused of the murder of her ex-husband. Cell tower data showed – according to the police – that she was home, when he died. She claimed to have been nearby but not at their common home. This story was backed by a tele-technician explaining that cell tower data cannot pinpoint the location of a suspect, only the area. The long-read about Mette Hansen also featured tele-engineers confirming the lack of precision in the cell tower data evidence along with several lawyers describing cases where their clients phones was registered more than 20 km from the clients position. The tele-engineers actually warned against using cell tower data as precise evidence.

Through surveys among the danish lawyers it was made clear that most of the lawyers didn’t know about the lack of precision in the cell tower data, which in turn could lead to weak defenses. Also the lawyers accused the prosecutors and police witnesses to keep the imprecision secret.

Two follow up stories was made. In one, several lawyers that the police and/or prosecutors/and or phone companies in several criminal cases handled the cell tower data wrongly causing mix-ups and errors. In the other, I got access to an internal police report describing how one of the biggest danish phone companies through a period of time delivered cell tower data containing serious errors.

During my research I found a related story which however doesn’t revolve around criminal cases. It turns out that the danish phone companies are logging cell tower data for internet trafic illegally. The story took the frontpage supplemented by a background article in Information, then reactions from politicians and the union for computer professionals.

If you wanna meet up during GIJC 2015, send me an email at – my gpg key is here. Feel free to email me with questions or tips regarding the use of cell tower data.