Authorities arrested a person suspected of sending the deadly poison ricin in an envelope addressed to the White House, a law enforcement source said on Sunday.
The letter was intercepted before it could be delivered.
In response to a Reuters query seeking confirmation of media reports that a woman accused of mailing the letter was taken into custody at the US-Canadian border, the FBI field office in Washington issued a statement saying: "An arrest was made of an individual allegedly responsible for sending a suspicious letter. The investigation is ongoing."
The law enforcement source told Reuters the "suspicious letter" was the same as the envelope found to contain ricin.
The source also said the person arrested was a woman with Canadian citizenship.
The Royal Canadian Mounted Police said on Saturday that it received a request for assistance from the FBI in the investigation and that the suspicious letter appeared to have been sent from Canada.
The RCMP said FBI analysis of a substance inside the envelope detected "the presence of ricin", a highly toxic agent derived from castor beans.
The toxin requires a deliberate act to convert it into a biological weapon, with exposure to an amount as small as a pinhead capable of killing someone within 36 to 72 hours. No known antidote exists.
Initially asked about the incident on Saturday, the FBI said it joined the US Secret Service and the US Postal Inspection Service in the investigation of "a suspicious letter received at a US government mail facility".
The envelope was intercepted at a US government mail centre before it could be delivered to the White House.
The Secret Service and the RCMP declined to comment on Sunday.
There have been numerous incidents involving envelopes mailed with ricin to US officials in recent years.
In 2018, a Utah man was indicted on charges of making ricin-related threats to President Donald Trump, FBI director Christopher Wray and other federal officials, with all the letters "containing castor bean material".
Two people were sentenced to prison in separate incidents of sending ricin-tainted letters to Barack Obama while he was president.
