An envelope addressed to the White House and intercepted by US authorities contained ricin, a deadly poison that appeared to have been sent from Canada, the Royal Canadian Mounted Police said on Saturday.
A spokesman for the force said it received a request for assistance from the FBI in connection with "a suspicious letter sent to the White House".
The force said analysis from the FBI found the substance in the envelope was ricin.
The envelope was intercepted at a government mail centre before it arrived at the White House.
The FBI said it was investigating the incident with support from the US Secret Service and the US Postal Inspection Service.
"At this time, there is no known threat to public safety," the FBI said.
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.
People can die within 36 to 72 hours if they are exposed to an amount as small as a pinhead. No known antidote exists.
There have been numerous incidents involving envelopes containing ricin being sent to US officials.
In 2018, a Utah resident, William Clyde Allen III, was indicted for making ricin-related threats, including mailing a threat against US President Donald Trump and federal officials including FBI director Christopher Wray, with all the letters "containing castor bean material. Mr Allen remains in custody.
In 2014, two people were convicted in separate incidents of sending ricin-tainted letters to Barack Obama, who was US president at the time.
In May that year, Mississippi resident James Everett Dutschke was sentenced to 25 years in prison after pleading guilty to sending letters with the deadly substance to Mr Obama, as well as a US senator and a state judge.
In July 2014, Texas actress Shannon Richardson was sentenced to 18 years for mailing letters containing ricin to Mr Obama and former New York mayor Michael Bloomberg.