By Edith Honan and Chris Francescani

NEW YORK (Reuters) - 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, 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 in Washington, D.C., on Sunday, indicated the presence of ricin.

"In both letters the content was identical," police spokesman Paul Browne said, adding that the packages contained "an oily substance" that was a pink or orange hue. "One letter was addressed to the mayor personally."

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

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

"In terms of why they've done it, I don't know. The letter ... obviously referred to our anti-gun efforts, but there's 12,000 people that are going to get killed this year with guns and 19,000 that are going to commit suicide with guns, and we're not going to walk away from those efforts," he said in remarks released by his press office.

Mayors Against Illegal Guns was founded in 2006, but the group's profile has been raised since the December 14 shooting at an elementary school in Newtown, Connecticut, that killed 20 children and six adults.

After that shooting, the group campaigned for bills that would expand the use of background checks for gun purchases and ban assault weapons, though both of those efforts were unsuccessful.

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.

Browne said previous letters sent to the mayor have tested positive for anthrax, though in most cases letters "with threats implying it was anthrax or ricin" contained only baking soda.

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

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 and Chris Francescani; Editing by Scott Malone, Paul Thomasch and Eric Beech)