A suspicious letter mailed to the White House was similar to two threatening, poison-laced letters on the gun control debate sent to New York Mayor Michael Bloomberg, one of the most potent advocates of stricter gun laws in the U.S., officials said Thursday.

The Secret Service said the letter was addressed to President Barack Obama and 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 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 one, a man in Washington state was charged with threatening to kill a federal judge in a letter that contained ricin. About a month earlier, letters containing the substance were addressed to Obama, a senator and a judge. Another man has been arrested in that case.

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 that all three letters apparently came from the same machine or computer and may be identical but 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.

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, Louisiana, 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.

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. (AP)