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At least some of them were not on the correct visas, and it's possible some may have overstayed.

The most egregious violation was that they were using Korean contractors for construction and there is no visa for that.

Also if you are going on a business trip, your company organises your visa for you. You cant really refuse, it's part of your job. So I feel quite sorry for these workers becuase they've been caught in the crossfire.