By Christinne Muschi

LONGUEUIL, Quebec (Reuters) – Canadian police on Monday searched an apartment in a Montreal suburb that is linked to the woman arrested for sending a ricin-filled envelope to the White House and to five other addresses in Texas, the Royal Canadian Mounted Police (RCMP) said.

U.S. authorities arrested a woman at the U.S.-Canada border near Buffalo, New York, on Sunday on suspicion that she sent the deadly poison by mail, addressed to the White House. The woman has joint Canadian and French citizenship, two sources said on Monday.

“We believe a total of six letters were sent, one to the White House and five to Texas,” RCMP officer Charles Poirier said outside the modern brown and grey building where the search was taking place. “We can’t confirm that she lived in (the apartment), but it is connected to her.”

Poirier could not say where in Texas the envelopes were mailed, but the police department in Mission, Texas, received a suspicious letter within the last week, Art Flores, a spokesman for the department, said. The department did not open the envelope and turned it over to the FBI, he said.

Flores also said the Mission police had arrested the woman now believed to be held in Buffalo in early 2019, but said he did not have records related to the arrest and referred further inquiries to the FBI.

The woman’s name has yet to be released.

The FBI is investigating several suspected ricin letters sent to law enforcement and detention facilities in South Texas, a U.S. law enforcement source told Reuters.

So far they have not found any link to political or terrorist groups, but the investigation is ongoing, the source said.

The RCMP’s special Chemical, Biological, Radiological, Nuclear, Explosives team is leading the operation, the RCMP said.

On Saturday, the RCMP confirmed the White House letter had apparently been sent from Canada and said the FBI had requested assistance.

The envelope was intercepted at a government mail center before it arrived at the White House.

Ricin is found naturally in castor beans but it takes a deliberate act to convert it into a biological weapon. Ricin can cause death within 36 to 72 hours from exposure to an amount as small as a pinhead. No known antidote exists.

(Reporting by Christinne Muschi in Longueuil, Steve Scherer in Ottawa and Mark Hosenball in Washington, additional reporting by David Ljunggren in Ottawa, writing by Steve Scherer; Editing by Chris Reese, Jonathan Oatis and Tom Brown)