They have to have evidence to secure a conviction and assuming they brought in equipment to try to track the source of the control signals which seemed to be stopping and starting regularly (going by how it's been reported) plus the fact most drone controls are about the same size as the ones for r/c cars so can move around it's going to be somewhat difficult.
As I said in another thread, maybe it's time the import and sale in the UK is stopped until there is a radar transponder fitted and a register set up.
Like any sensible investigation all possibilities are concerned however:
Eye witnesses included police and airport workers
No sightings were spotted after military tech was deployed
A downed drone was found near the perimeter
Im sure more info will be released as it becomes available