How might police track missing Megan Stammers?

Police in England and France are continuing their search for 15-year-old Megan Stammers, who is believed to be with a maths teacher from her school in Eastbourne.

As part of the investigation, CCTV images of Megan with Jeremy Forrest, 30, aboard a ferry from Dover to Calais on Thursday have been released.

Roger Gray, a former superintendent with the Metropolitan Police, spoke to the BBC to explain how modern technology may be used to provide clues to their whereabouts.