WASHINGTON — A woman suspected of having mailed an envelope containing ricin to the White House was arrested this weekend while trying to enter the United States from Canada, according to a federal law enforcement official.

Customs and Border Protection agents detained the woman, who had a firearm, as she tried to enter Buffalo, New York, the official said. No further information about her was immediately available.

The Joint Terrorism Task Force in Washington has been leading the investigation into who sent an envelope containing ricin to the White House, addressed to President Donald Trump, in recent days, as well as other envelopes containing ricin sent to a sheriff’s office and a detention facility in Texas.

Once discovered, the substance in the envelopes was confirmed to be ricin, a poison that is part of the waste produced when castor oil is made and has no known antidote.

Law enforcement officials said this weekend that the letters could have been sent from Canada, but that it was not clear when they were sent. The mail was intercepted last week, before it reached its destination.

No links to any international terrorist groups have been found, according to two law enforcement officials.

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The FBI did not immediately respond to a request for comment.

NBC News previously reported that the suspect had been detained.

The envelope addressed to Trump was intercepted at the final off-site processing facility before mail is sent to the White House mail room, according multiple law enforcement officials. All mail sent to the White House and other federal agencies in the Washington area is irradiated by the Postal Service and sorted in a facility that samples the air for suspicious substances.

This is the second time someone has tried to send ricin to Trump. In 2018, federal authorities intercepted mail suspected of containing ricin that was addressed to Trump and to top Pentagon and other national security officials. The Justice Department determined that the suspect, a Navy veteran named William Clyde Allen, had sent castor beans instead of ricin. He was charged in a seven-count federal indictment for threatening to use a biological toxin as a weapon.