This 1998 published book is part of the Hippo Mystery series. I vaguely remember these books from years ago and as I do love old books I snapped this up when I found it at a charity shop last year.
Friends Laura, Bethan and Gary are doing a school project at an animal park but it's not long before they discover something sinister going on there.
With a mystery to keep the reader intrigued this was one I really liked and I will be looking for more in this series.