Some evidence points to rogue telepaths.
Other evidence points to medical researchers who might have a motivation for needing to understand how telepathy works. And still other evidence doesn't seem to make sense. So one pair is sent to follow the same route as the last pair to be attacked. I read it while waiting for my car's timing belt to be replaced, which is just the sort of circumstance under which you'd want a book like this around. The death of God left the angels in a strange position.