A woman suspected of sending an envelope addressed to the White House, containing the poison ricin, has reportedly been arrested at the US-Canada border.
Officials told The Associated Press the woman was taken into custody by US Customs and Border Protection officers at the Peace Bridge border crossing near Buffalo, New York.
She is expected to face federal charges.
The letter had been intercepted earlier this week before it reached the White House.
The Royal Canadian Mounted Police said the letter addressed to the White House appeared to have originated in Canada.
It was intercepted at a government facility that screens mail addressed to the White House and President Donald Trump, and a preliminary investigation indicated it tested positive for ricin, according to the officials.
It is not the first time US presidents and officials have been targeted with ricin sent through the mail.
Federal investigators launched an immediate investigation to determine where the enveloped originated and who posted it. The FBI, the Secret Service and the US Postal Inspection Service were leading the probe.
In a statement, the FBI said agents were working to investigate “a suspicious letter received at a US government mail facility” and that there is “no known threat to public safety”.
Ricin used in previous threats
There have been several prior instances in which US officials have been targeted with ricin sent through the mail.
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.
The letters were intercepted, and no one was hurt.
In 2014, a Mississippi man was sentenced to 25 years in prison after sending letters dusted with ricin to President Barack Obama and other officials.