Spencer Platt / Getty Images file New York City Mayor Michael Bloomberg

New York City Mayor Michael Bloomberg was sent an anonymous threatening letter that preliminary tests indicated contained the presence of the poison ricin, law enforcement officials told NBC News.

A similar letter, which early tests indicated contained ricin, was also sent to the director of Mayors Against Illegal Guns, in Washington, D.C.

In both letters, the writer made threatening comments about Bloomberg’s support for gun control, NYPD Deputy Commissioner for Public information Paul Browne said. The letter to Bloomberg was opened on Sunday, and the letter in Washington was opened on Friday.

Law enforcement sources said the letters were postmarked May 20, from Shreveport, Louisiana.

Civilian personnel in New York and Washington who came in contact with the opened letters remain asymptomatic, officials said. However, members of the NYPD Emergency Service Unit who came in contact with the letter that was opened at the city's mail facility on Gold Street in Manhattan on Friday are being examined for minor symptoms of ricin exposure that they experienced on Saturday, which have since abated.

The FBI Joint Terrorism Task Force and the NYPD Intelligence Division, which is responsible for the Mayor's protection, are investigating the threats.

Additional reporting by Jonathan Dienst