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

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

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

The Trump administration is yet to comment on the reports.

The 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 September 19.

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One official told the New York Times that investigators believe the package was sent from Canada. Reports say the presence of ricin was identified after two tests.

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. According to the CDC, if a person is exposed to ricin, death can take place within 36 to 72 hours, depending on the dose received.

The CDC said ricin – 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.

In 2014, a Mississippi man was sentenced to 25 years in prison for sending letters dusted with ricin to former President Barack Obama and other officials.

In 2018, a former Navy veteran was charged with sending toxic letters to the Pentagon and White House.