system
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In this case they did … 
@QEHNick
This has downloader capability and may be responsible for the trz##.tmp files. There are several tools we could run against it but I still recommend SDFix first.
Is there any chance of keeping those USB drives away from the PCs for now? They will just continually re-infect them (if I’m not mistaken files transfers from an infected USB drive carries some risk even with autorun turned off).
No, it doesn’t. Nor does it make sense to allow the infection to continue while a solution to the scanning dilemma is being worked on.
It looks like you’ve had this problem since at least 22 March and, while I have every confidence if the ability of the analysts to solve the scanning dilemma, a two pronged approach seems logical.