WASHINGTON (AFP) - A suspicious letter addressed to the White House and that is "similar" to an apparently poison-laced letter sent to New York Mayor Michael Bloomberg has been intercepted, the Secret Service said on Thursday.
"The White House mail screening facility intercepted a letter addressed to the White House that was similar to letters previously addressed to Mayor Bloomberg in New York," Secret Service spokesman Edwin Donovan said.
"This letter has been turned over to the FBI Joint Terrorism Task Force for testing and investigation," Mr Donovan said in a short statement sent to Agence France-Presse.
Two letters sent last week - one to Mr Bloomberg and one to an official with the gun laws group he helped found, Mayors Against Illegal Guns - appeared to be laced with ricin, according to preliminary tests, the New York police said.
Mr Bloomberg told CBS late on Wednesday that the letter sent to him was "obviously" meant to show opposition to his lobbying for tighter gun laws.
The letters contained "anonymous threats" and "when tested locally, preliminarily indicated the presence of ricin", New York Police Department spokesman Paul Browne said. "The writer, in letters, threatened Mayor Bloomberg, with references to the debate on gun laws."
Mr Browne said the police officer who handled the letters had displayed minor symptoms of ricin exposure last Saturday that had since been addressed.
Mr Bloomberg, 71, the third-term mayor of New York City and Forbes magazine's 13th-wealthiest person with a fortune of US$27 billion (S$34 billion), has spearheaded the movement to tighten gun laws following last December's Newtown school massacre.
He has vowed to take on the nation's powerful gun lobby, infuriating United States conservatives, who also object to his campaign to ban the sale of extra-large soft drinks, accusing him of government overreach.
Mr Bloomberg's lobbying group plans to spend US$12 million on a national advertising campaign to counter the National Rifle Association (NRA).
He said he was "not angry" about the letters, and that the incident did not change the deadly realities of mass gun ownership in the US.
"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," he said.
"I know I speak for all of the close to 1,000 mayors in the mayors' coalition against guns. This is a scourge on the country that we just have to make sure that we get under control and eliminate."
The killing of 20 children and six adults in the Newtown elementary school shooting rampage in Connecticut led to widespread calls for tighter gun laws.
But under pressure from the NRA, Congress failed to pass even heavily watered-down legislation drafted in the wake of the tragedy.