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Associated Press
FILE - In this Jan. 14, 2013 photo, New York City Mayor Michael Bloomberg speaks at a gun violence summit at Johns Hopkins Bloomberg School of Public Health in Baltimore, where he outlined his proposals for federal gun control reforms. Police said Wednesday, May 29, 2013, that threatening letters containing traces of the poison ricin were opened Friday, May 24, 2013 at New York City?s mail sorting facility and Sunday, May 26, 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. (AP Photo/Patrick Semansky, File)

NEW YORK — Two threatening letters sent to Mayor Michael Bloomberg in New York and his gun-control group in Washington contained traces of the deadly poison ricin, police said Wednesday.

The anonymous letters were opened in New York on Friday at the city's mail facility in Manhattan and in Washington on Sunday at an office used by Mayors Against Illegal Guns, the nonprofit started by Bloomberg, police said.

Chief New York Police spokesman Paul Browne said preliminary testing indicted the presence of ricin in both letters but that more testing would be done. He said the threats contained references to the debate on gun laws and an oily pinkish-orange substance.

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The billionaire mayor has emerged as one of the country's most potent gun-control advocates, able to press his case with both his public position and his private money.

The people who initially came into contact with the letters showed no symptoms of exposure to the poison, but three officers who later examined the New York letter experienced some minor symptoms that have since abated, police said.

Browne would not comment on what specific threats were made or where the letters were postmarked, and would also not say whether the letters were believed to be linked to any other recent ricin cases.