Complex sentence: The disadvantage of the system is that it provides a single point of failure for security: if the key to a district's Knox Boxes is stolen or copied, a thief can enter any building that has a Knox Box.
Split into simpler sentences: The disadvantage of the system is that it provides a single point of failure for security. If the key to a district's Knox Boxes is stolen or copied, a thief can enter any building that has a Knox Box.