A team of investigators paid for by the British government is gathering evidence to use against the leaders of the Islamic State if they are ever tried for war crimes. The BBC has been told that investigators are piecing together evidence on the militant group's most senior members. Frank Gardner has more.
It has no known address and until now it's avoided all publicity, but this team of highly experienced war crimes investigators is piecing together files on up to 400 of the most senior members of Islamic State, known as IS. The team has sources both inside Syria and in neighbouring countries, some operating at extreme personal risk. They smuggled documents, memory sticks, witness testimonies; they built up an intricate picture of how IS is run. But getting suspects out of a war zone will not be easy, and as yet there is no court ready to try them.