A woman suspected of sending a letter addressed to President Donald Trump found to contain the lethal poison ricin has been arrested, according to US law enforcement officials.
Officials say the woman was taken into custody by US Customs and Border Protection officers as she tried to enter the US from Canada at a border crossing in New York state on Sunday.
US prosecutors are expected to bring charges against her. Her name was not immediately released.
The letter was intercepted before reaching the White House, the Federal Bureau of Investigation (FBI) said on Saturday.
It was opened at an off-site government facility where mail bound for President Trump and other White House staff is screened.
The Royal Canadian Mounted Police said the letter appeared to have originated in Canada.
Ricin is a poison naturally found in castor beans and can be deadly to humans – exposure to a quantity as small as a pinhead can kill an adult within 36 to 72 hours.
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Similar envelopes have been sent to the White House on several previous occasions.
A Navy veteran was arrested in 2018 and confessed to sending envelopes to Mr Trump and members of his administration that contained the substance from which ricin is derived.
Authorities said the man, William Clyde Allen III, sent the envelopes with ground castor beans to the president and a number of other top officials, including FBI director Christopher Wray.
Others targeted were the then-defence secretary Jim Mattis, then-CIA director Gina Haspel, Admiral John Richardson, who at the time was the Navy’s top officer, and then-Air Force secretary Heather Wilson.
Four years prior, a Mississippi man was sentenced to 25 years in prison after sending letters dusted with ricin to the then-president Barack Obama and other officials.