Canadian police on Monday searched a home in the Montreal area in connection with a letter addressed to US President Donald Trump that is believed to have contained the poison ricin.
The operation, undertaken at the request of the FBI, comes after a female suspect was arrested trying to cross into the US from Canada, a spokesman for the Royal Canadian Mounted Police said.
The woman, who was carrying a firearm when she was taken into custody, was expected to appear in a US court on Monday to face federal charges.
The special RCMP unit -- experts in chemical, biological, explosives and nuclear threats -- searched a home in Longueuil, south of Montreal.
Police refused to say if the home belonged to the suspect.
Ricin, which is produced by processing castor beans, is lethal even in minute doses if swallowed, inhaled or injected, causing organ failure.
The letter was discovered last week and did not reach the White House, according to The New York Times and CNN.
Mail addressed to the White House is first inspected and sorted in depots outside Washington.
CNN said the contents of the envelope were tested repeatedly at one depot and confirmed to contain ricin.