Retributions - Lena's Friends, #4

When bombs start going off around Bristol and the West Country, it falls to Lena's friends to help the police to stop the murders. The victims seem to have no connection to each other, so why are they being targeted. Answering that question will go a long way towards solving the crimes. And why does the anonymous killer keep sending emails to the police? Can Lena and her friends find the killer before the number of murder victims increases?