Answer:
Yes, it was caused by an insider or outsider.
Step-by-step explanation:
Yes, I believe that this event was caused by an insider or outsider. This is because, if a USB removable flash drive is attached to all the office computers by an insider and now an outsider sends a mail to the company’s General group mail; if the mail has a link that contains some infected worms and virus; once anybody clicks on the link in the mail then, all the computers will be affected by the virus.
Therefore, this event may be caused by the insider who clicked the infected link or by the outsider sent the infected link in the mail.