Police said preliminary testing indicated the presence of ricin in both letters involving Bloomberg, but that more testing would be done. Police said the threats contained references to the debate on gun laws and an oily pinkish-orange substance.
Last month, authorities in Washington intercepted a letter addressed to Obama that was found after testing to contain ricin. The letter was similar to one mailed to Mississippi Republican Sen. Roger Wicker, which also tested positive for ricin.
Federal authorities last month arrested a Mississippi man suspected of sending the ricin-laced letters to Obama, Wicker and a Mississippi judge.