To throw the book at someone means to charge them with as many crimes as possible, but if they only found the upper torsos of the bodies then there wouldn't be enough evidence to charge him with any crimes.
Consider the statement "Brother number two claimed the police couldn't charge him with any crimes at all because they only found the upper torsos of his butchered bodies.". Is the statement "Brother number two claimed the police couldn't throw the book at him because they only found the upper torsos of his butchered bodies." false? Yes/no/unknown