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.
"In terms of why they've done it, I don't know," Bloomberg said at an event Wednesday night.
One of the letters "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," said Bloomberg, adding that he didn't "feel threatened."
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 by phone Wednesday, adding that the firm would do whatever needed to ensure safety.
A mayor's spokesman also speaking for the nonprofit said he had no comment.
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.
Separately, Bloomberg also has made political donations to candidates who share his desire for tougher gun restrictions. His super PAC, Independence USA, put $2.2 million into a Democratic primary this winter for a congressional seat in Illinois, for example. Bloomberg's choice, former state lawmaker Robin Kelly, won the primary and the seat.
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