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.
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Ricin has been addressed to the White House on several previous occasions.
A Navy veteran was arrested in 2018 and confessed to sending envelopes to Mr Trump and members of his administration that contained the substance from which ricin is derived.
Authorities said the man, William Clyde Allen III, sent the envelopes with ground castor beans to the president and a number of other top officials, including FBI director Christopher Wray.
Others targeted were the then-defence secretary Jim Mattis, then-CIA director Gina Haspel, Admiral John Richardson, who at the time was the Navy’s top officer, and then-Air Force secretary Heather Wilson.
Four years prior, a Mississippi man was sentenced to 25 years in prison after sending letters dusted with ricin to the then-president Barack Obama and other officials.