WASHINGTON — A suspicious letter mailed to the White House and intercepted this week was similar to two threatening, poison-laced letters on the gun law debate sent to New York Mayor Michael Bloomberg, one of the nation's most potent gun-control advocates, officials said Thursday.

Yet another letter became known publicly on Thursday, one tainted with the poison ricin and mailed to President Barack Obama from Spokane, Wash., the FBI said. Authorities have arrested a man in Spokane in connection with that letter, which was intercepted May 22.

The Secret Service said the White House-bound letter similar to the ones Bloomberg was sent was intercepted by a White House mail screening facility. Two similar letters postmarked in Louisiana and sent to Bloomberg in New York and his gun control group in Washington contained traces of the deadly poison ricin.

It wasn't immediately clear whether the letter sent to Obama contained ricin. It was turned over to the FBI's Joint Terrorism Task Force for testing and investigation.

The two Bloomberg letters, opened Friday in New York and Sunday in Washington, contained an oily pinkish-orange substance.

New York Police Department Commissioner Raymond Kelly said Thursday the same machine or computer had produced the two letters to Bloomberg and the similar one to Obama and that they may be identical. He referred specific questions to the FBI.

The FBI said in a statement that field tests on the letters were consistent with the presence of a biological agent, and the letters were turned over to an accredited laboratory for the kind of thorough analysis that is needed to verify a tentative finding. "More letters may be received," the statement said, without elaboration.

The body of the letter mailed to New York was addressed to "you" and referenced the gun control debate. Kelly said the unsigned letter says, in so many words: "Anyone who comes for my guns will be shot in the face." He refused to quote directly from the letter, saying he didn't want to do the author's bidding.

Bloomberg has emerged as one of the country's most important gun-control advocates, able to press his case with both his public position and his private money.

The New York letter was opened at the city's mail facility in Manhattan in a biochemical containment box, which is a part of the screening process for mayor's office mail.

"In terms of the processes and procedures that are in place now we think they worked," Kelly said. "This is sort of an effect of the post-9/11 world that we live in that these checks and facilities are in place and the system worked."

The second letter was opened Sunday by Mark Glaze, director of Mayors Against Illegal Guns, the Washington-based nonprofit Bloomberg started.

The letter Glaze opened tested positive for ricin initially. The other letter to Bloomberg at first tested negative but tested positive at a retest Wednesday.

The postal workers union, citing information it got in a Postal Service briefing, said the letters bore a Shreveport, La., postmark. Kelly would not comment on the origin of the letter.

Louisiana State Police spokeswoman Julie Lewis said state authorities have deferred to the FBI and have not opened an investigation. The Shreveport postal center handles mail from Louisiana, Texas and Arkansas, so the letter could have come from any of those states, Lewis said.

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. The mayor visited the mailroom on Thursday but made no public comments on the topic.

On Wednesday, he said he didn't know why they were sent.

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.

According to the federal Centers for Disease Control and Prevention, 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.

The letters were the latest in a string of toxin-laced missives, but authorities would not say whether the letters to Bloomberg and Obama were believed to be linked to any other recent case.

In Washington state, a 37-year-old was charged last week with threatening to kill a federal judge in a letter that contained ricin. On Thursday, the FBI said a suspicious letter containing ricin was mailed to Obama from Spokane on the same day similar ricin-tainted letters were mailed to the judge and to a post office. A fourth letter, sent to nearby Fairchild Air Force Base, continues to undergo testing, officials said.

About a month earlier, letters containing the substance were addressed to Obama, a U.S. senator and a Mississippi judge. One of the letters postmarked in Memphis, Tenn., was traced back to Tupelo, Miss., and a Mississippi man was arrested.

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 – have stalled.

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.

___

Associated Press Writers Jennifer Peltz and Frank Eltman in New York, Holbrook Mohr in Jackson, Miss., and Pete Yost in Washington contributed to this report.

Also on HuffPost:

Loading Slideshow...
  • Obama Is Not An American Citizen

    One of the most popular Obama conspiracy theories (aka the Birther Movement), endorsed (or not denied) by the likes of <a href="http://www.huffingtonpost.com/_263532.html">Tom Delay</a>, <a href=http://www.huffingtonpost.com/_251393.html>Lou Dobbs</a>, <a href="http://www.huffingtonpost.com/_250891.html">Chuck Norris</a> and <a href="http://www.huffingtonpost.com/_242555.html">Liz Cheney</a>, it claims that Obama is not an American citizen, thus rendering his presidency illegitimate, or something. The theory hinges on the false claim that his birth certificate is fake. Birther queen Orly Taitz (a lawyer and dentist from California) continues to lead the march, despite her lawsuit getting <a href="http://www.huffingtonpost.com/2009/10/29/orly-taitz-lawsuit-thrown_n_338870.html">thrown out</a> and a <a href="http://www.huffingtonpost.com/2009/10/13/orly-taitz-slapped-with-2_n_318546.html">$20,000 fine</a>.

  • Obama Wants "Re-Education Camps For Young People

    Rep. Michele Bachmann (R-Minn.), a proponent of many bizarre and outlandish critiques of President Obama, declared in April that the White House was trying to install <a href="http://www.huffingtonpost.com/2009/04/06/bachmann-obama-wants-re-e_n_183552.html">youth re-education camps</a>. "It's under the guise of -- quote -- volunteerism. But it's not volunteers at all. It's paying people to do work on behalf of government," said the Minnesota Republican. "I believe that there is a very strong chance that we will see that young people will be put into mandatory service. And the real concerns is that there are provisions for what I would call re-education camps for young people, where young people have to go and get trained in a philosophy that the government puts forward and then they have to go to work in some of these politically correct forums."

  • Larry Sinclair's Rabbit Hole Of Insanity

    This one is a doozy. Larry Sinclair, <a href="http://www.politico.com/news/stories/0608/11164.html">a petty criminal specializing in forgery</a>, spread wild accusations via YouTube involving <a href="http://www.huffingtonpost.com/2008/06/18/obama-accuser-larry-sincl_n_107900.html">gay sex, drugs and possible murder committed by Barack Obama</a>. At one point reporters apparently took Sinclair seriously enough to attend a press conference he gave, most remarkable for his lawyer's appearance in a kilt, which he explained was intended to secure comfort for his unusually large sexual organs: "Those at the other end of the spectrum find [pants] quite confining."

  • The Case Of The Fake Thesis

    In which a group of right-wingers get taken in by a satiric blog post on Barack Obama's college thesis, written ages ago, and frantically presented as real. The post, which was tagged as SATIRE, described how <em>Time</em> Magazine's Joe Klein had obtained ten pages of a thesis written by Barack Obama, in which he wrote about how the Constitution was "inherently flawed." Then Rush Limbaugh and Michael Ledeen presented this story as <a href="http://www.huffingtonpost.com/2009/10/23/fake-obama-thesis-story-g_n_332255.html">fact</a>.

  • Bill Ayers Wrote 'Dreams From My Father"

    A popular theory on right-wing websites like <a href="http://www.freerepublic.com/focus/news/2101151/posts">The Free Republic</a>, <a href="http://www.americanthinker.com/2008/10/who_wrote_dreams_from_my_fathe_1.html">American Thinker</a> and <a href="http://www.wnd.com/?pageId=110784">WorldNetDaily</a>, claiming that educator (and former radical) Bill Ayers helped Obama write "Dreams Of My Father." There is <a href="http://www.obamaconspiracy.org/category/books/">no evidence</a> supporting this claim.

  • Obama As Antichrist

    Is Obama the Antichrist? <a href="http://www.youtube.com/watch?v=HDB5BFoSQLs&feature=player_embedded">These</a> <a href="http://www.snopes.com/politics/obama/antichrist.asp">folks</a> think maybe so. McCain also verged on endorsing this theory during the campaign season with an attack ad called "<a href="http://www.time.com/time/politics/article/0,8599,1830590,00.html">The One.</a>"

  • Obama As Muslim

    A theory that began circulating during the campaign season and has been kept alive by Tea Partiers, and other ultra-conservatives. It's taken different forms over the year (<a href="http://www.scribd.com/doc/6516016/Will-Obama-Swearin-on-the-Holy-Koran-or-the-Holy-Bible-After-Attacking-Jesus-Sermon-on-the-Mount">Will Obama be sworn into office on Qur'an??</a>, asks one website.) Most recently Tea Party leader Mark Williams had a meltdown on CNN, and <a href="http://www.huffingtonpost.com/2009/09/15/tea-party-leader-melts-do_n_286933.html">claimed</a> that Obama was an "Indonesian Muslim" and a "welfare thug." According to Obama and those who know him, this is not so.

  • Tells America In His Acceptance Speech To "Serve Satan"

    Ever listened to Led Zeppelin's "Stairway to Heaven" backwards? How about Obama's presidential acceptance speech? This video suggests that the speech, like the Zeppelin song, contains a Satanic message when played in reverse. Go ahead - take a bong hit, try it and tell us what you hear.

  • Hugo Chavez Funded The Obama Campaign

    Did Hugo Chavez fund the Obama campaign? No. But some people thought so after a Fox News affiliate issued a false report claiming that <a href="http://www.snopes.com/politics/obama/chavez.asp">he did</a>.

  • Michelle Banished Barack's Girlfriend To The Caribbean

    One of the few conspiracy theories to go after the Obamas' marriage. The claim is that Michelle banished Barack's female aide (and suspected lover) to the Caribbean. Every person involved, including the woman, says this is <a href="http://www.dailymail.co.uk/news/worldnews/article-1076695/Obama-hit-affair-smears-following-claims-attractive-aide-banned-wife.html">false</a>.

  • Obama Is Malcolm X's Son

    Initiated by the author of the <a href="http://atlasshrugs2000.typepad.com/atlas_shrugs/2008/10/how-could-stanl.html">Atlas Shrugged</a> blog and claiming that Obama is actually Malcolm X's son. As Ezra Klein <a href="http://www.prospect.org/csnc/blogs/ezraklein_archive?month=10&year=2008&base_name=obama_x">pointed out</a> at the time: "Honestly, who wouldn't give Malcolm props for that one?" Klein continued: "And think there's anyway we can fit Tupac into the bloodline? Or maybe Obama actually is Tupac? You know that whole shooting was some bullshit, right? Incidentally, I see no reason to stop with Malcolm. History is full of great Xs for Barack Obama to be related to. Professor X, which would explain why millions of white people lose their minds in his presence. Weapon X, which would explain the adamantium grafted onto his skeleton. XML, which would explain why it's so easy to structure his data. I'm sure there are others."