The letter was opened at a government facility where mail bound for Donald Trump and other staff is screened.
An envelope addressed to the White House which was intercepted before it reached its destination has been found to contain a lethal poison.
Officials said ricin was discovered inside the letter, which was opened at a government facility where mail bound for President Donald Trump and other staff is screened.
Ricin is a poison naturally found in castor beans and can be deadly to humans – exposure to a quantity as small as a pinhead can kill an adult within 36 to 72 hours.
In a statement, the FBI said agents were working to investigate “a suspicious letter received at a US government mail facility” and that there is “no known threat to public safety”.
The FBI is leading the probe alongside the Secret Service and the US Postal Inspection Service.
US media including The New York Times and CNN have reported that the letter was sent from Canada.
The White House and Secret Service declined to comment.