An envelope sent to the White House has been found to contain a deadly poison, US officials have confirmed.
The letter was intercepted at a facility which screens mail addressed to Donald Trump.
A preliminary investigation indicated that it contained ricin – a poison found naturally in castor beans, the official said.
Federal investigators are now working to determine who posted the potentially lethal missive.
The FBI, the Secret Service and the US Postal Inspection Service are leading the investigation.
In a statement released on Saturday, 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”.
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 to the president, FBI director Christopher Wray and the then-defence secretary Jim Mattis, among others.
The letters were intercepted and no one was hurt.
In 2014, a Mississippi man was sentenced to 25 years in prison after sending letters dusted with ricin to President Barack Obama and other officials.