US authorities have intercepted an envelope addressed to US President Donald Trump that contained a substance identified as the poison ricin, US media reported on Saturday.
The letter was discovered earlier this week and did not reach the White House, according to the New York Times and CNN.
The Times said it was believed that the letter was mailed from Canada.
Mail addressed to the White House is first inspected and sorted in depots just outside Washington.
CNN said the contents of the envelope were tested repeatedly at one depot and confirmed to contain ricin.
"The FBI and our US Secret Service and US Postal Inspection Service partners are investigating a suspicious letter received at a US government mail facility," the FBI's Washington field office said in a statement.
"At this time, there is no known threat to public safety."
Authorities contacted by AFP declined to discuss additional details of the incident.
Ricin, which is produced by processing castor beans, is lethal even in minute doses if swallowed, inhaled or injected, causing organ failure.
It has no known antidote.