Sentence:  `` Unfortunately '' , says Chief Postal Inspector David H. Stephens , who has prosecuted many device quacks , `` the ghouls who trade on the hopes of the desperately ill often cannot be successfully prosecuted because the patients who are the chief <WSD>witnesses</WSD> die before the case is called up in court '' . 
Meaning of the word marked with <WSD> tags:  a close observer; someone who looks at something (such as an exhibition of some kind)