Members of the U.S. Secret Service Uniformed Division secure an area in front of the White House in Washington, D.C., on April 17, 2013 as a part of tightened security following the Boston marathon bomb blasts. (Getty Images)
The letter is being tested for ricin. It was received Thursday at an off-site facility, and did not reach the White House, sources said.
The text of that mailing was identical to the letters sent to the mayor and his gun group, which threatened: "what's in this letter is nothing compared to what I've got planned for you," police and law enforcement sources said.
The FBI is investigating, according to CBS News. The letter was reportedly postmarked Shreveport, La., the same as the letters sent to Bloomberg and his Mayors Against Illegal Guns organization.