For U.S. Postal Service employees, the disclosure that an envelope addressed to Sen. Roger Wicker, R-Miss., may have contained a poison could revive unnerving memories of the 2001 anthrax attacks that killed two workers at the Brentwood mail processing plant in Washington, D.C. According to USA Today, the envelope is undergoing further analysis to confirm the presence of the toxin known as ricin. Here’s what the Postal Service is saying so far, in a statement provided by spokesman Dave Partenheimer this morning. “The U.S. Postal Service is working diligently with authorities to determine if there was in fact a hazardous substance inside an envelope addressed to a U.S. senator, and, if so, what type…