Police in America say two anonymous letters addressed to New York City Mayor Michael Bloomberg and his gun control group contained material believed to be the potentially deadly poison ricin.

The New York Police Department also said that the letters referenced the debate on gun laws.

Emergency workers who came in contact with the letters initially showed minor symptoms of ricin exposure.

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.

Bloomberg said he did not feel either threatened or angry by the letters and that they would not affect his work on gun control.