NEW YORK - Two letters addressed to New York City Mayor Michael Bloomberg and his gun control group contained material believed to be the deadly poison ricin, and referenced the debate on gun laws, police said on Wednesday.

The New York Police Department said initial tests on the two letters, opened in New York on Friday and Washington, D.C. on Sunday, indicated the presence of ricin.

New York Mayor Michael Bloomberg gets on a bicycle as he poses for photographers during a launch event for a new public bike rental program in New York, May 27, 2013. [Photo/Agencies]

Emergency workers who came in contact with the letters initially showed minor symptoms of ricin exposure, the police said. Those symptoms have since abated. Civilian personnel in New York and Washington who came in contact with the opened letters showed no symptoms of ricin exposure.

The Washington letter was opened by Mark Glaze, the director of Mayors Against Illegal Guns, a group founded by Bloomberg that lobbies for stricter gun laws. The other letter was opened at a mail facility in Manhattan.

Both contained threats against Bloomberg and mentioned the gun debate, police said in a statement.

The FBI Joint Terrorism Task Force and the NYPD Intelligence Division are investigating the incident.

The discovery of the letters comes just weeks after ricin-tainted letters were mailed to President Barack Obama and other government officials. James Everett Dutschke, 41, a martial arts instructor, was arrested in Tupelo, Mississippi, on April 27 on suspicion of mailing those letters.

Ricin is a lethal poison found naturally in castor beans, but it takes a deliberate act to convert it into a biological weapon. Ricin can cause death within 36 to 72 hours from exposure to an amount as small as a pinhead. No known antidote exists.

(Reporting by Edith Honan; Editing by Scott Malone, Paul Thomasch and Richard Chang)