Two anonymous letters sent to the New York City Mayor Michael Bloomberg contained material believed to be the deadly poison ricin and referenced the debate on gun laws, police said.
Initial tests on the letters - one of which was opened in New York on Friday and the other in Washington D.C on Sunday - indicated the presence of the poison, the New York Police Department said.
Police spokesman Paul Browne said the content in both letters was "identical" and that the packages contained "an oily substance."
"One letter was addressed to the mayor personally," he added.
Emergency workers who came in contact with the letters initially showed minor symptoms of ricin exposure but have recovered now, police said.
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