MONTREAL, CANADA — Canadian police on Monday searched a home in the Montreal area in connection with a letter addressed to US President Donald Trump that is believed to have contained the poison ricin.

The operation, undertaken at the request of the FBI, comes after a female suspect was arrested trying to cross into the United States from Canada, a spokesman for the Royal Canadian Mounted Police said.

The woman, who was carrying a firearm when she was taken into custody, was due to appear in a US court Tuesday to face federal charges after the hearing was delayed a day.

The special RCMP unit — experts in chemical, biological, explosives and nuclear threats — searched a home in Longueuil, south of Montreal.

Police refused to say if the home belonged to the suspect, but the RCMP added that ricin-containing letters were sent to five other addresses in Texas.

Police in Hidalgo County Texas, without referring directly to the White House case, said ricin-laced envelopes had been sent to four officers.

“At this time due to a active federal investigation I cannot make any further comments,” Hidalgo Sheriff Eddie Guerra wrote on Twitter, adding one of the letters was addressed to him and that no one was injured.

Canadian media named the suspect as 53-year-old Pascale Ferrier, who is Canadian and French.

Ricin, which is produced by processing castor beans, is lethal even in minute doses if swallowed, inhaled or injected, causing organ failure.

The Trump letter was discovered last week and did not reach the White House, according to The New York Times and CNN.

Mail addressed to the White House is first inspected and sorted in depots outside Washington.

CNN said the contents of the envelope were tested repeatedly at one depot and confirmed to contain ricin.