A suspicious letter addressed to President Barack Obama and similar to ricin-laced ones sent to New York Mayor Michael Bloomberg has been intercepted by a White House mail screening facility, the secret service has said.
The letter has been turned over to the FBI's Joint Terrorism Task Force for testing and investigation.
White House spokesman Josh Earnest said on Thursday that Mr Obama was aware of the letter addressed to him.
New York Police Department Commissioner Raymond Kelly said the letters apparently all came from the same machine.
The letters sent to Mr Bloomberg contained an oily pinkish-orange substance. It was not clear if the missive to Mr Obama contained the same substance.
The body of the letter was addressed to "you" and referenced the gun law debate.
Mr Kelly said the letters say, in so many words: "Anyone who comes for my guns will be shot in the face." He refused to quote directly from the letters, saying he did not want to do the author's bidding.
Two threatening letters postmarked in Louisiana and containing traces of the deadly poison ricin were sent to Mr Bloomberg in New York and to his gun-control group in Washington, officials said.
The anonymous letters sent to Mr Bloomberg 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 non-profit started by Mr Bloomberg, police said.
Testing indicated the presence of ricin in both letters.
Mr Kelly would not comment on the origin of the letter.The postal workers' union, citing information it got in a Postal Service briefing, said the letters bore a Shreveport, Louisiana, postmark.
Louisiana State Police spokeswoman Julie Lewis said state authorities have deferred to the FBI and have not opened an investigation.
The Shreveport postal centre handles mail from Louisiana, Texas and Arkansas, so the letter might have come from any of those states, Lewis said.
The mayor did not respond to questions about the letters on Thursday morning as he arrived for and left a speech to the Real Estate Board of New York.
The billionaire mayor has emerged as one of the most potent US 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.