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

The letter was discovered at a screening facility for White House mail earlier this week, the BBC quoted the officials as saying to the outlets on Saturday.

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They said a substance found inside the envelope was identified as ricin, a poison found naturally in castor beans.

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.

“At this time, there is no known threat to public safety,” the FBI told CNN in a statement on Saturday.

One official told the New York Times that investigators believe the package was sent from Canada.

The Royal Canadian Mounted Police (RCMP) said on 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 swallowed, inhaled or injected, can cause nausea, vomiting, internal bleeding and ultimately organ failure, the BBC reported.

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).