A letter containing the deadly poison ricin that was mailed to the White House may have originated in Canada, Canadian law enforcement has said.

Canadian authorities are working with their US counterparts on the investigation.

The material was addressed to US President Donald Trump. The envelope was intercepted at a government facility that screens mail addressed to the White House and Trump, a US official said.

A preliminary investigation indicated it tested positive for ricin, a poison found naturally in castor beans, the official said.

The Royal Canadian Mounted Police (RCMP) is cooperating in this investigation with US authorities, the media outlet Global News reported.

It quoted a spokesperson for RCMP as saying that “initial information” from the investigation “suggests that the letter originated in Canada.”

The identity of the sender has not been revealed as yet.

A spokesperson for Bill Blair, Canada’s minister for public safety and emergency preparedness, also confirmed that there were “concerning reports of packages containing ricin directed toward US federal government sites.” Investigators in both countries are “working closely” on the matter, the spokesperson added.

This is not the first time that the deadly poison was sent to US presidents or top American officials.

A US Navy veteran was arrested in 2018 and confessed to sending envelopes to Trump and members of his administration that contained the substance from which ricin is derived.

Authorities said at the time that William Clyde Allen III sent the envelopes with ground castor beans to the president, FBI Director Christopher Wray, along with then-defence secretary Jim Mattis, then-CIA director Gina Haspel, top naval officer Adm John Richardson and then-Air Force secretary Heather Wilson. The letters were intercepted, and no one was hurt.

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

(With inputs from Agencies)