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AUDIE CORNISH, HOST:
From NPR News, this is ALL THINGS CONSIDERED. I'm Audie Cornish.
MELISSA BLOCK, HOST:
And I'm Melissa Block. Over years of conflict in Syria, it's estimated that thousands of Westerners have turned up to join the fight. And now some are crossing into Iraq, as well. Last week, an English-language recruitment video from the Sunni extremist group ISIS, the Islamic State of Iraq and Syria, went viral.