There is absolutely no way that any "invenstigation" or security check takes (or should take) 10 business days. Its way more likely that they are stalling for time.
I know its too much time and when i win something i will want it as soon as possible.But you should give respect to the site owners as well.If they asked 10 days ok fine wait 10 days and let's see what happens? if it were me i would be that panic and say about the same thing here and there again and again.i hope this issue resolved without any problem and as they paid more than that to a single winner i don't think there is anything to worry about it.