Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: Washington (CNN) As the sun rose in northern Germany Tuesday, authorities swooped in and carried out a dramatic raid that captured three Syrian men believed to be members of an ISIS sleeper cell with ties to Paris terror attacks network. The men, who range in age from 17 to 26, came into Europe along the same migrant route refugees are using to escape the brutal Syrian civil war. These latest arrests have highlighted tensions in Europe over ISIS' ability to exploit these routes to infiltrate Western cities. The latest arrests come after other similar cases were revealed in a recent CNN investigation.Europe has taken steps to improve vetting, but the continent is facing a major security challengeThe US is better able to control the flow of refugees entering the country 
 Question:ISIS has used migrant routes to move operatives into _.

Europe