A postal distribution center where four anthrax-laced letters passed through two years ago was fumigated with chlorine dioxide gas as crews wound up the decontamination process this weekend .
Workers cleaning up a postal distribution center where four letters laced with anthrax had passed through two years ago planned to complete the fumigation operation early Sunday .
Shares of Schering-Plough closed down 4 cents at $ 16.76 in Thursday trade on the New York Stock Exchange .