Authorities have intercepted a threatening letter addressed to President Barack Obama that was "similar" to a ricin-laced mail sent to New York Mayor Michael Bloomberg, according to the US presidential security unit.
A Secret Service official said on Thursday that the letter was sent to the White House, and was intercepted at its screening facility.
"This letter has been turned over to the FBI Joint Terrorism Task Force for testing and investigation," Secret Service spokesman Edwin Donovan said in a statement.
Another letter sent to Bloomberg appeared to contain the highly toxic substance ricin, according to preliminary tests, New York police said on Wednesday.
In recent weeks, authorities have stopped several ricin-laced letters.
A Tupelo, Mississippi, man was charged last month with sending poison-laced letters to Obama and two other public officials.
Everett Dutschke was being held on charges of developing and possessing ricin and attempting to use it as a weapon, according to the Justice Department.
'Opposition to tighter gun laws'
Separately, federal authorities last week arrested a Washington state man, Matthew Ryan Buquet, who they suspect mailed a letter containing ricin to a US district judge in Spokane.
Buquet, 37, was charged with mailing threatening communications.
On Thursday, the FBI's Seattle office said in a statement that a similar letter containing ricin was sent from Spokane to Obama and postmarked May 13.
The letter Buquet is accused of sending to the judge in Washington state was postmarked the same day.
Bloomberg, the Mayor of New York, had told CBS on Wednesday that the letter sent to him was "obviously" meant to show opposition to his lobbying for tighter gun laws.
Like President Obama, Bloomberg has also advocated for tighter gun control laws after December's Newtown school massacre.
Bloomberg's lobbying group plans to spend $12 million on a national advertising campaign to counter the National Rifle Association.