Two threatening letters containing traces of the deadly poison ricin were sent to Mayor Michael Bloomberg in New York and his gun-control group in Washington, 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.