Advice on investigating a Minecraft crime

This is probably the weirdest post this sub has ever seen, but I need advice. A crime has been committed on a Minecraft server I administrate. I need help in investigating. Especially conducting interviews.

It's a case of griefing. The server's playerbase numbers around 50, and consists of students at my high school. How on earth should I go about finding out who the culprit is? I just need a general bullet-point list.

Thank you so much. 50 players are counting on me.