By what means does Shakespeare build suspense before the Ghost's appearances? What disturbing political events happen in the background of Act I?

Before the play even begins, we find that the Ghost has appeared to some of Hamlet's friends, without speaking. The characters in Act I, scene i are agitated and have summoned Hamlet to come and see. The Ghost appears twice onstage, wordlessly, before he appears to Hamlet. The mere existence of a Ghost would create great suspense for a superstitious Elizabethan audience. His wordless appearances serve to build the anticipation.

Prior to the opening of the play, Hamlet's father, the King of Denmark, has been assassinated, but no one knows the identity of the murderer. The king's brother, Claudius, has married Hamlet's mother and taken over the throne.