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Police have appealed for information to trace any "associates" of the Westminster attacker, Khalid Masood.

The group which calls itself Islamic State have described him as their 'soldier'.

Charlie Winter, senior research fellow at the International Centre for the Study of Radicalisation and Political Violence, told Mark Mardell "the age that we live in is one of unsophisticated tactics but high-impact terrorism."

He said this kind of attack is "much more difficult to defend against" but is a "microcosm of what the terrorist threat that we face in the UK is today."

(Photo: A policeman standing by flowers outside the Houses of Parliament. Credit: PA)