WASHINGTON (AP) - 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. The mayor visited the mailroom on Thursday but made no public comments on the topic. According to the federal Centers for Disease Control and Prevention, ricin is a poison found naturally in castor beans. 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. 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.