Foreshadowing is a technique used by writers where future events in a novel or story are suggested or hinted at by events in the present. This text uses foreshadowing in a number of different places in order to hint at the answer or solution to future mysteries. One example of this is the presence of the bomber who had already been bombing various apartments. Note how foreshadowing is used in the following quote:

[Mrs. Wexler] did not want anything blowing up during Angela's party. But the Wexler apartment was exactly where the bomber planned to set the next bomb.

Note how this foreshadows the event when Turtle opens the package and Angela directs it towards her as a firework comes out and it cuts her cheek. This quote also foreshadows, rather ironically, the identity of the bomber, as it points towards Angela being the bomber as it is her who chooses to sabotage her own party for her own purposes. To find other examples of foreshadowing look at the ways in which certain clues or hints are given about the identity of certain people or the solving of certain mysteries.