Authorities intercepted a threatening letter addressed to US President Barack Obama that was similar to ones sent to New York Mayor Michael Bloomberg, the Secret Service said on Thursday.
Letters sent to Bloomberg and his gun control group contained material believed to be the deadly poison ricin and contained a reference to gun control, New York police said on Wednesday.
A Secret Service official said the White House letter was similar but did not elaborate.
The White House mail screening facility turned the letter over to the FBI's Joint Terrorism Task Force, the Secret Service official said on condition of anonymity.
Authorities have stopped several ricin-laced letters in recent weeks.
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.
Authorities also intercepted a letter sent to Mississippi Senator Roger Wicker that preliminary tests showed contained ricin.
Last week, federal authorities arrested a man in Washington state last week on charges of mailing a letter containing ricin to a US district judge in Spokane.