New York City Mayor Michael Bloomberg
New York City Mayor Michael BloombergAFP file

Anonymous letters sent to New York City Mayor Michael Bloomberg and the director of the organization Mayors Against Illegal Guns, in Washington, D.C., contained material that initially tested positive for ricin, the NYPD announced.

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 -- a nonprofit founded by Bloomberg that lobbies for stricter gun control, a police statement said Wednesday, according to Fox News.

Both letters are believed to have threatened the mayor and made reference to the gun control debate.

They reportedly contained an oily pinkish-orange substance, said the NYPD.

The police department would not release the specific threats or where the letters were postmarked.

Civilian personnel who came into contact with the letters were not exhibiting symptoms of ricin exposure. However, three officers who came into contact with the letter sent to Bloomberg initially experienced some minor symptoms, Fox News reported.

The incidents are being investigated by the FBI Joint Terrorism Task Force and the NYPD Intelligence Division.