According to New York Times sources, the exact location of the origins of the letter is unknown, but believe to have been sent from Canada. Mail sent to the White House is screened off site before reaching the complex.
According to the CDC, Ricin is a poison found naturally in castor beans.
“If made into a partially purified material or refined into a terrorist or warfare agent, ricin could be used to expose people through the air, food, or water,” the CDC said, adding that there is no known antidote for ricin.
The White House has been the target of ricin mailings in the past. In 2013, the FBI confirmed two letters were sent to the White House containing ricin.