WASHINGTON • The authorities in the US have arrested a person suspected of sending the deadly poison ricin in an envelope addressed to the White House but intercepted before it could be delivered, a law enforcement source has said.

In response to a Reuters query seeking confirmation of media reports that a woman accused of mailing the ricin-tainted letter had been taken into custody at the US-Canadian border, the Federal Bureau of Investigation office in Washington issued a statement on Sunday saying: "An arrest was made of an individual allegedly responsible for sending a suspicious letter."

The statement added: "The investigation is ongoing."

The law enforcement source, familiar with the case but speaking on condition of anonymity, told Reuters the "suspicious letter" referred to in the FBI statement was the same as the envelope found to have contained ricin. The source also said the person arrested was a woman of Canadian citizenship.

The Royal Canadian Mounted Police (RCMP) said last Saturday that it had received a request for assistance from the FBI in the investigation and that the suspicious letter in question appeared to have been sent from Canada.

The RCMP further stated that an FBI analysis of a substance inside the envelope detected "the presence of ricin", a highly toxic agent derived from castor beans.

While naturally occurring, 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 causing death within 36 to 72 hours.

No known antidote exists.

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 both declined comment on Sunday.

There have been numerous incidents involving envelopes with ricin sent 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 have been convicted and sentenced to prison in separate incidents of sending ricin-tainted letters to Mr Barack Obama while he was president.

REUTERS