Washington DC: A package containing ricin poison that was addressed to US President Donald Trump has been intercepted before it reached the White House, officials told US media.

The letter was discovered at a screening facility for White House mail earlier this week, the officials said.

Two tests were done to confirm the presence of ricin. All mail for the White House is sorted and screened at an offsite facility before reaching the White House.

A US law enforcement official told CNN that investigators are looking into the possibility the ricin package sent to Trump came from Canada.

Mary-Liz Power, chief spokeswoman for Canada's Minister of Public Safety Bill Blair, said in a statement, "We are aware of the concerning reports of packages containing ricin directed toward US federal government sites. Canadian law enforcement is working closely with their US counterparts. As this is an active investigation we cannot comment further."

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The Federal Bureau of Investigation (FBI) and the Secret Service are investigating where the package came from and whether others have been sent through the US postal system.

Ricin is produced by processing castor beans. It is a lethal substance that, if swallowed, inhaled or injected, can cause nausea, vomiting, internal bleeding and ultimately organ failure.

No known antidote exists for ricin. If a person is exposed to ricin, death can take place within 36 to 72 hours, depending on the dose received, according to the US Centers for Disease Control and Prevention (CDC).

The CDC said the poison - which has been used in terror plots - can be manufactured into a weapon in the form of a powder, mist or pellet.

The White House and other federal buildings have been the target of ricin packages in the past.