US Secret service are investigating a letter that contained deadly poison ricin which was addressed to the White House. Law enforcement officials said that the letter was detected at an off-site facility that screens mail addressed to the White House. The letter tested positive for the poison both in field test and a laboratory test.

“The FBI and our U.S. Secret Service and U.S. Postal Inspection Service partners are investigating a suspicious letter received at a U.S. government mail facility,” the FBI said in a statement on Twitter. “At this time, there is no known threat to public safety.”

However, the letter was not sent to the White House as it was intercepted at the screening facility. According to a media report, the letter was sent from Canada.

According to a media report, there were other letters too that tested positive for ricin that were discovered elsewhere, but this one was addressed to the US President Donald Trump.

Ricin is a highly potent toxin derived from castor beans, and is extremely lethal if inhaled or injected, but less so if ingested. Inhaling a dose the size of a few grains of salt can be lethal to adults.

It may be recalled that actress Shannon Richardson, who appeared on The Walking Dead, was convicted in 2014 of sending envelopes containing ricin addressed to then President Barack Obama and New York City Mayor Michael Bloomberg. She was sentenced to 18 years in prison.