"An average of some 400,000 Pakistani Brits a year fly back to the old country for vacation or to visit their relatives. From the airports in Karachi, Lahore and Islamabad, where they land, side trips to the madrassas — Koranic schools — where they were originally radicalized, or to a terrorist training camp in the tribal areas that straddle the Pakistani-Afghan border, go undetected.
There is no way to keep track of thousands of passengers arriving from the United Kingdom every day. Nor can MI5 cope with up to 1,000 a day returning to their U.K. homes from trips to Pakistan."
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Of course, Australia doesn't have a similar problem, even if on a smaller scale..