Letters sent to New York City Mayor Michael Bloomberg and Mark Glaze, director of the group Mayors Against Illegal Guns, contained material that preliminary testing indicates is ricin, according to New York City Police Department Deputy Commissioner Paul Browne.

The letters contained anonymous threats with references to the national debate over gun regulation, Browne said. The letter to Bloomberg was opened on Friday and tested both in New York and at the National Bioforensic Analysis Center in Maryland; both tests indicated the presence of ricin. Members of the NYPD  Emergency Service Unit were treated Saturday for minor symptoms of ricin exposure. 

Bloomberg co-founded Mayors Against Illegal Guns with Boston Mayor Thomas Menino and has used the group in recent weeks aggressively target lawmakers in both parties who voted against expanded background checks.

The FBI Joint Terrorism Task Force and the NYPD Intelligence Division are investigating the threats to Bloomberg. The Washington Blade reported earlier this week that the FBI and Fire Department investigated a suspicious envelope mailed to Glaze.

Letters containing ricin surfaced recently in Washington, D.C. and Spokane, Wash.

The FBI alleges that Mississippi martial arts instructor J. Everett Dutschke sent ricin-laced letters to President Obama and Sen. Roger Wicker of Mississippi. Matthew Ryan Buquet, a 37-year-old janitor, has been charged with sending the letters in Spokane.