Mounties tell CityNews Montreal they are executing a search warrant at a residence linked to the mail sent to the White House, as well as packages sent to five locations in Texas.
The incident recalls prior instances in which US officials have been targeted with ricin.
The RCMP said a team with expertise in handling chemicals and explosives is leading the investigation to ensure the safety of the public.
An RCMP investigation is now underway at Vauquelin Boulevard, a residential street located south of Montreal in St-Hubert, Que.
Canadian police on Monday swooped on an apartment complex in Montreal in connection with a ricin-laced letter that was sent to Trump, allegedly by a woman who then tried to enter the USA while armed with a gun. Local police and fire units are also on-site.
Canadian police on Monday are conducting an operation near Montreal airport in connection to the ricin-contaminated letter, the Quebec section of the Royal Canadian Mounted Police (RCMP) said in a statement on Twitter. The woman was taken into custody by US Customs and Border Protection officers at the Peace Bridge border crossing near Buffalo and is expected to face federal charges, three law enforcement officials told The Associated Press. "At this time, there is no known threat to public safety".
The federal police force is acting on a request from the U.S. Federal Bureau of Investigations after a Canadian woman was arrested at the Canada-U.S. border Sunday night.
RCMP spokesman Dan Brien said Saturday that initial information from the investigation suggests the letter originated in Canada. Those envelopes contained a substance from which ricin is derived. The letters were intercepted, and no one was hurt.
Authorities arrested on Sunday a person suspected of sending the deadly poison by mail, addressed to the White House.
Law enforcement sources told Associated Press on Monday that the same woman is also suspected of mailing ricin-laced letters to government agencies in the southern tip of Texas, southeast of Laredo.