Two threatening letters sent to New York City's mayor and his group that advocates for gun control contained traces of the deadly poison ricin, police said on Wednesday.
The anonymous letters were opened in New York on Friday at the city's mail facility and in Washington on Sunday at an office used by Mayors Against Illegal Guns, a nonprofit started by billionaire Michael Bloomberg to counter the powerful US gun lobby.
Both the letters contained threats to Bloomberg and an oily pinkish-orange substance, New York Police Department spokesman Paul Browne said.
He would not comment on what specific threats were made or where the letters were postmarked. He also wouldn't say whether investigators believe they were sent by the same person.
Bloomberg said one of the letters referred to his "anti-gun efforts." He also said he did not feel threatened.
Browne said preliminary testing indicted the presence of ricin in both letters but that more testing would be done. A mayor's spokesman, also speaking for the nonprofit, said he had no comment.
Bloomberg has been one of the country's most visible gun control advocates since the December shooting of 20 young children and six adults at a Connecticut school with a legally purchased, high-powered rifle.
Bloomberg's group lobbies lawmakers and counts more than 700 mayors nationwide as members.
Bloomberg's separate political action committee supports political candidates who support gun control - an effort to counter the National Rifle Association, which pressures politicians to follow its point of view.
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 abated, police said.
Word of the letters comes about a month after letters containing ricin were addressed to President Barack Obama, a US senator and a Mississippi judge. A Mississippi man was arrested in that case.
Browne would not comment on what specific threats were made or where the letters were postmarked. He also wouldn't say whether they were handwritten or typed and whether investigators believe they were sent by the same person.
According to the Centers for Disease Control, ricin is a poison found naturally in castor beans. Symptoms can include difficulty breathing, vomiting and a redness on the skin depending on how the affected person comes into contact with the poison.