A woman suspected of sending an envelope containing the poison ricin, which was addressed to White House, has been arrested at the New York-Canada border, three law enforcement officials told The Associated Press.
The woman was taken into custody by US Customs and Border Protection officers and is expected to face federal charges, the officials said.
The letter addressed to the White House appeared to have originated in Canada, the Royal Canadian Mounted Police have said.
The officials were not authorised to discuss the ongoing investigation publicly and spoke on condition of anonymity.
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All packages addressed to the White House are sorted and screened at a secure offsite facility prior to delivery.
The envelope to the White House was caught at the final offsite processing facility where mail is screened before being sent to the White House mail room.
Lab tests confirmed the presence of ricin - which is deadly if inhaled - in the letter and an FBI investigation was launched to locate the sender.
"The FBI and our US Secret Service and US Postal Inspection Service partners are investigating a suspicious letter received at a US government mail facility," the FBI said in a statement Saturday.
"At this time, there is no known threat to public safety."
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.