NEW YORK (AP) — New York City police say two anonymous letters sent to Mayor Michael Bloomberg contained traces of the deadly poison ricin.

Police said Wednesday that the threatening letters were opened Friday in New York at the city's mail facility and Sunday in Washington at the headquarters of the nonprofit started by Bloomberg, Mayors Against Illegal Guns. Both were addressed to Bloomberg and contained threats referencing the debate on gun laws.

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Police say preliminary testing indicated the presence of ricin.

Police say civilian personnel who came into contact with the letters were not showing any symptoms, but some officers who later examined the one in New York were.

Ricin is a poison found naturally in castor beans.

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It's not clear if the letters were related to other threatening, ricin-laced letters sent to other lawmakers recently.