A package containing ricin poison addressed to US President Donald Trump was intercepted before it reached the White House, officials told US media.
The letter was discovered at a White House mail screening facility earlier this week, officials said.
They said a substance found inside the pouch was identified as ricin, a poison that occurs naturally in castor beans.
The Trump administration has yet to comment on the reports.
The Federal Bureau of Investigation (FBI) and the Secret Service are investigating where the package came from and whether others were sent via the US postal system.
“At this time, there is no known threat to public safety,”
An official told the New York Times that investigators believe the package was sent from Canada. Reports say the presence of ricin was confirmed after FBI tests.
The Royal Canadian Mounted Police (RCMP) said Saturday it was working with the FBI to investigate the “suspicious letter sent to the White House”.
Ricin is produced by processing castor beans. It is a lethal substance that, if ingested, inhaled or injected, can cause nausea, vomiting, internal bleeding and eventually organ failure.
There is no known antidote for ricin. If a person is exposed to ricin, death can occur within 36 to 72 hours, depending on the dose received, according to the United States Centers for Disease Control and Prevention (CDC).
The CDC said the poison – which has been used in terrorist plots – can be turned into a weapon in the form of a powder, mist or ball.
The White House and other federal buildings have been the target of ricin packages in the past.
In 2014, a Mississippi man was sentenced to 25 years in prison for sending ricin-dusted letters to former President Barack Obama and other officials.
Four years later, in 2018, a former Navy veteran was accused of sending toxic letters to the Pentagon and the White House.