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 have said.
The anonymous letters were opened in New York on Friday at the city's mail office and in Washington on Sunday at an office used by Mayors Against Illegal Guns, a non-profit group started by billionaire Michael Bloomberg to counter the powerful US gun lobby.
Both the letters contained threats to Mr 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, or say whether investigators believe they were sent by the same person.
Mr 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 non-profit group, said he had no comment.
Mr 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.
His group lobbies politicians and counts more than 700 mayors nationwide as members.
Mr Bloomberg's separate political action committee backs 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.
News 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.
Mr Browne would not say whether the letters were believed to be linked to any other recent ricin cases. Federal officials were investigating.
According to the Centres 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.