In the third scenario, roughly similar, the gang did not pass by the withdrawal zone between the fourth event and the abandonment of the car. ‘This supposition arose from the fact that only four hours had passed between the last crime and the moment the police found the car,’ explains Marie Trotta. ‘It was too short a time lapse to imagine a withdrawal between the two events.’ A fourth scenario, finally, offered two round trips between the withdrawal zone and the second crime, in other words a robbery at a supermarket, thus explaining a possible prior reconnaissance of the site. But in adding on this extra round trip the possible routes greatly exceeded 100 kilometres. The researchers’ attention thus focused on the third scenario.

‘From this scenario,’ explain the two researchers, ‘different possible itineraries were established by adding up the cost surfaces in order to retain only pixels close to 100 kilometres as possible withdrawal sites. But we still had too many possible withdrawal sites, too many candidate pixels.’

…to discovering plausible withdrawal zones

To reduce the number of candidate pixels, it was thus necessary to integrate further data beyond that of distance. After consultation with the police, two other factors were taken into account. ‘When criminals choose a withdrawal zone, they tend to favour rural zones,’ explains Jean-Paul Kasprzyk. ‘They also privilege sites close to main roads, so that they can flee rapidly.’