WASHINGTON — A suspicious letter mailed to the White House 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.

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 known 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 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 a 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 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 mail sent to the mayor’s office.