WASHINGTON (REUTERS) - An envelope addressed to the White House contained a substance identified as ricin, a deadly poison, several media outlets reported on Saturday (Sept 19), citing a federal official briefed on the matter.

The envelope believed to have come from Canada was intercepted at a government mail center before it arrived at the White House, the New York Times, CNN and the Wall Street Journal reported.

Asked about the reports, the FBI said the agency and "US Secret Service and US Postal Inspection Service partners are investigating a suspicious letter received at a US government mail facility".

"At this time, there is no known threat to public safety."

The White House and US Secret Service declined to comment.

Ricin is found naturally in castor beans but it takes a deliberate act to convert it into a biological weapon.

Ricin can cause death within 36 to 72 hours from exposure to an amount as small as a pinhead.

No known antidote exists.

There have been numerous incidents involving envelopes mailed with ricin to US officials.

In 2018, a Utah man, William Clyde Allen III, was indicted for making ricin-related threats, including mailing a threat against Trump and other federal officials including FBI director Christopher Wray, with all the letters "containing castor bean material."

Allen remains in custody.

Two people were convicted in separate incidents of sending ricin-tainted letters to then-President Barack Obama.

In May 2014, a Mississippi man, James Everett Dutschke, was sentenced to 25 years in prison after pleading guilty to sending letters with the deadly substance to Obama, as well as a US senator and a state judge.

In July 2014, a Texas actor was sentenced to 18 years for mailing letters containing ricin to Obama and former New York Mayor Michael Bloomberg.