The letter has been turned over to the FBI's Joint Terrorism Task Force for testing and investigation.
It was unclear how the letter, intercepted Wednesday by the White House facility, was similar to the ricin-laced ones addressed to Bloomberg.
Two threatening letters postmarked in Louisiana and containing traces of the deadly poison ricin were sent to Bloomberg in New York and to his gun-control group in Washington, officials said.
The anonymous letters were opened in New York on Friday at the city's mail facility in Manhattan and in Washington on Sunday at an office used by Mayors Against Illegal Guns, the nonprofit started by Bloomberg, police said Wednesday.
Police said preliminary testing indicated the presence of ricin in both letters involving Bloomberg, but that more testing would be done. Police said the threats contained references to the debate on gun laws and an oily pinkish-orange substance.
Last month, authorities in Washington intercepted a letter addressed to Obama that contained a "suspicious substance." This letter was similar to one mailed to Mississippi Republican Sen. Roger Wicker, which tested positive for ricin.
The anonymous letters were opened in New York on Friday at the city's mail facility in Manhattan and in Washington on Sunday at an office used by Mayors Against Illegal Guns, the nonprofit started by Bloomberg, police said.
Chief New York Police spokesman Paul Browne said preliminary testing indicted the presence of ricin in both letters but that more testing would be done. He said the threats contained references to the debate on gun laws and an oily pinkish-orange substance.
The billionaire mayor has emerged as one of the country's most potent gun-control advocates, able to press his case with both his public position and his private money.
The people who initially came into contact with the letters showed no symptoms of exposure to the poison, but three officers who later examined the New York letter experienced minor symptoms that have since abated, police said.
Browne would not comment on what specific threats were made or where the letters were postmarked. He also wouldn't say whether they were handwritten or typed and whether investigators believe they were sent by the same person.
The letters were the latest in a string of toxin-laced missives. In Washington state, a 37-year-old was charged last week with threatening to kill a federal judge in a letter that contained ricin. About a month earlier, letters containing the substance were addressed to President Barack Obama, a U.S. senator and a Mississippi judge. A Mississippi man was arrested in that case.
Federal officials and NYPD were investigating. Browne would not say whether the letters were believed to be linked to any other recent ricin cases.
Police said the letter in Washington, D.C., was opened by Mark Glaze, the director of Mayors Against Illegal Guns. He was working out of the offices of The Raben Group, a Washington lobbying firm where he keeps an office. Glaze happened to open the letter while sitting outside over the Memorial Day weekend, said the firm's founder, Robert Raben.
"I'm very concerned about our employees and co-workers and clients. I'm sorry that we live in a world in which people do such awful things. Thank God, right now, everybody's physically fine," Raben said Wednesday, adding that the firm would do whatever needed to ensure safety.
According to the Centers for Disease Control, ricin is a poison found naturally in castor beans. Symptoms can include difficulty breathing, vomiting and redness on the skin depending on how the affected person comes into contact with the poison.
Bloomberg and Boston Mayor Thomas Menino founded Mayors Against Illegal Guns, which now counts more than 700 mayors nationwide as members. It lobbies federal and state lawmakers, and it aired a spate of television ads this year urging Congress to expand background checks and pass other gun-control measures after the school shooting in Newtown, Conn. The background check proposal failed in a Senate vote in April, and other measures gun-control advocates wanted -- including a ban on sales of military-style assault weapons -- went by the wayside.
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