What is the suspenseful event surrounding Sister Sandrine, in Chapter 31?

Although Silas kills Sister Sandrine, in Chapter 31, things are suspenseful because the Sister was able to make a series of phone calls before she died. In the calls, she conveyed the message that a group of people is dead. Presumably, that means that Silas has killed everyone who may know the location of the keystone. The fact that the author doesn't reveal who the Sister called adds more suspense to the story, though, since the reader doesn't know what will happen next or how that will come into play.