What's behind the latest spate of ricin mailings?

In recent days, threatening letters laced with the deadly poison ricin have been mailed to New York City Mayor Michael Bloomberg (I), the office of his gun control advocacy group, and possibly President Obama.

On Thursday, the Secret Service confirmed that it had intercepted a letter similar to the ones mailed to Bloomberg in New York and to his nonprofit's Washington, D.C., office. The service said the letter had been intercepted at a D.C. sorting facility and sent away for further testing, so it was not immediately known if that letter, like the others, was also laced with ricin.

The letters addressed to Bloomberg and Mark Glaze, director of his advocacy group, Mayors Against Illegal Guns, reportedly demanded that they stop pushing for tougher gun control laws. "What's in this letter is nothing compared to what I've got planned for you," the letters warned.

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The letters, according to CNN, also read:

The text of those letters was identical to that in the letter sent to Obama, according to NBC.

Law enforcement officials have not said if they have any suspects yet. However, the American Postal Workers Union said on its website that the two letters were postmarked May 20 in Shreveport, La.

Bloomberg said he didn't "feel threatened" by the letters, adding that "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."

It's the third time in two months that ricin-laced letters have been sent to prominent government officials.

In April, a Mississippi man allegedly tried to frame an Internet enemy by sending ricin-tainted letters to Obama and Sen. Roger Wicker (R-Miss.) That man, James Everett Dutschke, was charged with using and possessing ricin, and he's being held without bond.

Then last week, a Washington man was arrested for allegedly mailing a poisoned envelope to a U.S. district judge in Spokane, Wash.

Ricin, a toxin made from processed castor beans, can be lethal when inhaled or ingested in large enough doses.