In a previous test one of my TAs discovered a fraudulent pattern in six of the exams. The answers were uncomfortably similar, actually identical in many places. I called the students to my office and brought two colleagues to help me "interview" them individually (ask them questions about the test, gauge their level of knowledge about the subject, ...)
One of the students reacted very explosively at my one and only question: "Could you please tell me how you solved 'problem 2'? He told me he felt threatened by the question and that I had no right to ask him questions after the exam. To what I reply: "do you or do you not know?"
At that point things spiraled very fast out of control, he reacted violently, stepping out of the office while muttering some insults. On the way out he hit the door. I tried to calm him down but he got only angrier and the color of his words became redder. After seeing this, the other students refused to continue the interviews with my colleagues and left.
I definitely don't know if I should have approached the situation in a different way. Should I make them repeat the exam? Start a disciplinary process?
In case of suspected misconduct I should bring the case, along with evidence, to the Dean. He then reviews it and decides whether to bring it to the Academic Council or stop it there. They ultimately have the power to decide what to do. However, almost a year ago a colleague also faced a situation that involved fraud during a test, and the Dean decided to not do anything because the potential punishment for the student would be too harsh (he might be expelled)
Just to add some details into the situation: At the moment of the incident I was talking with one student, and each of my colleagues was doing the same with a student. The six of us were in the same (big) room. And the other three students were waiting outside.