A package containing the lethal poison ricin was mailed to US President Donald Trump at the White House, last week.
The package was discovered at a screening facility for White House mail.
The Federal Bureau of Investigation (FBI) and the Secret Service are investigating where it came from.
There are unconfirmed reports that it was posted in Canada. The Royal Canadian Mounted Police (RCMP) said on Saturday it was working with the FBI.
Ricin is a lethal substance derived from castor beans. It causes death by internal bleeding and organ failure.
There is no known antidote.