A letter addressed to Mayor Bloomberg arrived late last week at a municipal building in New York, and a second letter containing the poison arrived at a building in Washington that houses Mayors Against Illegal Guns, a group Bloomberg helps run and finances.

　　The anonymous letters sent to Bloomberg were opened last Friday, and preliminary tests on both letters concluded that they contained ricin, according to local media reports.

　　It added that the writer of the letters made reference to debate on gun laws. Mayor Bloomberg is a strong supporter of gun control and he is the co-chairman of the group Mayor Against Illegal Guns.

　　New York Police Department (NYPD) Emergency Service Unit officers who came into contact with the letter at the city mail facility on Gold Street in Manhattan have been treated for minor symptoms of ricin exposure.

　　The FBI Joint Terrorism Task Force and the NYPD Intelligence Division have launched an investigation.

　　Last month, ricin-tainted letters addressed to President Barack Obama, as well as U.S. Sen. Roger Wicker and an 80-year-old Mississippi judge, were intercepted.

　　Derived from the castor oil plant, ricin is a highly toxic protein that can kill a full-grown adult in a dose the size of just a few grains of salt.