A woman has been arrested after allegedly sending a poisoned letter to Donald Trump.
The suspect - who has not been identified - was taken into custody by Customs and Border Protection agents at a border crossing in New York on Sunday, an official told CNN.
She is said to have posted a package containing ricin to the White House addressed to the president.
Washington DC prosecutors are expected to charge the woman, understood to have been armed when she was stopped by police, the official added.
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All items addressed to the White House are screened off site prior to being sent over to presidential residence's mail room - with law enforcement managing to intercept the package last week, the FBI said on Saturday.
The Bureau launched an investigation to track down the sender after lab tests confirmed potentially deadly ricin was in the letter.
A statement said: "The F.B.I. and our U.S. Secret Service and U.S. Postal Inspection Service partners are investigating a suspicious letter received at a U.S. government mail facility."
"At this time, there is no known threat to public safety."
The suspect was attempting to re-enter the US from Canada when she was arrested, and The Royal Canadian Mounted Police has confirmed it is working alongside the FBI.
The White House and U.S. 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.
Ricin can cause death within 36 to 72 hours from exposure to an amount as small as a pinhead. No known antidote exists.
There have been numerous incidents involving envelopes mailed with ricin to US officials.
In 2018, a Utah man, William Clyde Allen III, was indicted for making ricin-related threats, including mailing a threat against Trump and other federal officials including FBI Director Christopher Wray, with all the letters "containing castor bean material."
Allen remains in custody.
Two people were convicted in separate incidents of sending ricin-tainted letters to then-President Barack Obama.
In May 2014, a Mississippi man, James Everett Dutschke, was sentenced to 25 years in prison after pleading guilty to sending letters with the deadly substance to Obama, as well as a US senator and a state judge.
In July 2014, a Texas actor was sentenced to 18 years for mailing letters containing ricin to Obama and former New York Mayor Michael Bloomberg.