I think the investigators would be where the suspects were...that's why SY are on Portugal...questioning suspects with the help of rogatory letters...
Suspects, persons of interest or witnesses?
Perseverance with the investigation has been not inconsiderable in time and endeavour of the proletariat.
Remind me sir how many people have thus far been charged with an offence resultant from all this toil?
Although I would concur, sir, the logical place to carry out the work is where the events took place and where the weightier portion of the papers are archived. Where the guilty reside is yet to be determined. Spain, Portugal, Holland, Australia, United Kingdom and any one of the Scandinavian countries all have been mooted as possible. A veritable United Nations assortment. Actually it is yet to be determined what happened, I don't recall the British or Portuguese authorities broadcasting that do you? Other than several possibilities of "lines of inquiry"
One hopes it will all be resolved adequately e'er long.